I will write down a few of my own understandings, ask a few questions, and request for some advice. Of course correct me if I'm wrong (likely). Please guys keep in mind that this is for me, a few of my friends, and all the newbies, so try not to go deep with details to keep it easier for us.
High on profile? Increase speed, or use speed breaks, OR both. Why not to increase V/S? Because it will increase speed. Correct?
If you have a speed restriction, and you use speed breaks with selected speed, autothrust will add power to keep speed, correct?
Generally whenever using speed breaks while flying on target selected or managed speed, autothrust will compensate with adding thrust?
What if you're ON profile coming for approach but a bit too fast. Use speed breaks to help decelerate?
Below profile? Decrease V/S, and autothrust compensates to keep speed same?
Max flap setting for speed breaks on 320 is flaps 3. 319 and 321 even when flaps full.
Happens often where you need to use speed breaks that late?
Apologies for some stupid questions, but this will be very helpful to us.
Whilst not rated on the bus, I can offer some general hints that I/we use on the 737 NG..
1) Above FL100 - wind up the speed to increase rate of descent to get back on profile. If you have descent or speed restrictions use in combination with speedbrake to achieved desired result.
2) Below FL100 - Wind speed off to around 220kts and use the speedbrake and flaps to increase rate. On the 737 we are instructed to 'avoid' using flaps as speedbrakes, but as long as your not too fast then sometimes you don't have any of options.
Some people use V/S to finesse exactly what they want in terms of descent management - me included. However, V/S (on the 737 at least) does not offer overspeed protection so it is sometimes frowned on.
If your below the profile you could use V/S to ease off the rate - we could alternatively use VNAV (the equivalent of Managed Path - I think) and it would command 1000ft/min until reacquiring the profile.
If you are above 230 kt use speed brakes. But only if you can't accelerate further, due to speed constraint.
Below 230 it's. Use gear. If you need to use speed rake as well, it's time to rethink your energy and reposition, unless it's only a reasonably small adjustment. I.e. less than 10kt to reach Vfe next or such like.
MY opinion is that speed brake is next to useless, under 220/230 kt.
A few answers to some of your questions, in my humble opinion!:
Yes if you're high on the profile initially you should increase speed, and then use speedbrake to increase rate of descent (RoD) further. If you need to increase past that, or are told to slow down then you can think about changing the config (flaps and gear). Which mode you use is up to yourself, Open descent is generally the best for increasing speed, as it will dive to reach the higher airspeed (make sure you keep an eye on TCAS as you don't want to trigger a TA/RA!) V/S can be used however at upper levels it generally isn't, it is used more as you get towards the ground and want to 'fine tune' your RoD.
As for the A/THR adding power against speedbrake, you need to full understand exactly how each of the modes work. Open descent will always have THR IDLE and will pitch for the speed. In this case, using the speedbrakes will simply increase the rate of descent, the power will ALWAYS be at THR IDLE. V/S pitches for the RoD that you have requested and the A/THR will do its best to match the speed. I.e if you have a high RoD then the A/THR will reduce to idle however the speed may well increase if the RoD is high enough. In this case the speedbrakes can be used to try and hold the speed back. If you have a low RoD selected then yes the A/THR will increase thrust to match the select/managed speed. If you use speedbrakes now then NOTHING will happen apart from the A/THR will increase thrust to keep the speed. The RoD will not change since the aircraft will pitch to maintain it no matter what the speedbrakes are doing.
Also you are correct in saying that the A320 cannot use speedbrake in conf Full, however the A321 cannot use the speedbrake in Conf 3 OR Conf Full. The A318 I believe can use it in all flap settings, but am not sure.
Basically as other people have said, above FL100 it is a good idea to just increase the speed, unless of course you have a speed limit! You can also use speedbrakes to help. Speedbrakes work much better at high speed.
Once below FL100 and are starting to slow down in the terminal area (it really isn't a good idea to come barrelling into the terminal area doing 320kts!) you can then consider using flaps. Conf 2, speed 180kts and the speedbrakes will give you a very good rate of descent, and will usually allow you to meet any speed restrictions ATC have given you. If you are still too high then you can throw the gear out.
Both the gear and speedbrakes can be used if you are too fast. It is quite common in a tailwind that you will struggle to slow the aircraft down, in this case select a low RoD and use the speedbrakes to reduce the speed. If you are below the profile then you can reduce the RoD as needed, the A/THR will keep the speed selected.
One great bit of advice is that you NEVER want to have a RoD that is equal to your current altitude. I.e if you are at 2000ft then DO NOT have a RoD of 2000fpm or more. You are 1 min away from hitting the ground which is never a good place to be!
Remember as well, if in doubt then ask for more track miles! There is no shame, and I know a lot of airports that have ask me to do impossible approaches! ATC often don't fully take into account the wind etc.
MrHorgy gives sound advice, and as another poster mentions, if you are running out of ideas, consider gear, the vast majority of new pilots end up high on the profile because they don't take care of the excess height when they need to, which is at the beginning of the descent, not within 30 NM of the field.
You use the terms High on Profile and Low on Profile, and seek advice for both situations.
Be sure to know the difference between the Profile as calculated by the FM(G)C and the "real" profile, that is, your altitude/speed combination in relation to your track miles to touchdown.
If you have tied the aircraft to the FM(G)C computed profile, then opening the speed window and increasing speed will indeed lead to Autothrust increasing thrust. (DESCEND mode in Airbus, VNAV PATH in Boeing).
If you are above the FM(G)C computed profile, but are not coupled to the path (OPEN DESCENT in Airbus or FL CH in Boeing), then increasing the speed in the open speed window will lead to a steeper dive, with thrust remaining in idle.
Whether the FM(G)C profile is identical to your real world profile, depends on what is in the FM(G)C legs data. Sometimes an arrival route will have an altitude restriction that is way above or below the ideal idle descent path towards the runway. Other times, the FM(G)C route is much longer or shorter than what you will get in real life from ATC, depending on the traffic situation.
In V/S mode, the vertical speed is primary, and is achieved via pitch control. Speed is then controlled by Autothrust. A/T cannot give less than idle, or more than (e.g.) climb thrust, so if you ask for enough vertical speed, A/T may not be able to keep the speed at the desired value.
Remember that all this info is given to you by the FMA's: the acronyms in their respective positions really mean something.
Finally, the devices are speedbrakes, not speedbreaks, a break is time off to have a cup of coffee in between lessons.