Hi,
Ignoring whatever your company manual dictates regarding configuration gates, I use the following as a safe guide:
"When would you generally slow down from 250 knots on an average day?(considering no tail wind)."
Be at S-speed when intercepting the glide. The speed tends to 'hang' or increase a bit when you go down at 3 degrees, so intercepting the glide any faster than about 200kts can make life a bit difficult.
This all depends on where your intercept point lies. 250kt to 20nm is fine if you are decelerating in the descent. You can start a descent at 15nm sometimes.
I use 250kt/20nm/6000' and then adjust based on 1nm - 300ft - 10kt (level decel). Work around that triangle to adjust.
"Would you prefer to come down with a high speed to your platform altitude, level, and then slow down and start configuring,
OR slow down whilst coming down and catching the glide path at the same time?"
It is more efficient to get level and then decelerate. However, high speeds and high rates of descent at low level can be a bit dodgy.
"Flying in yesterday I was going to intercept the GS at 5000 feet, so I decided to slow down and configure. I realise I was about 16 miles away, and when I asked for flaps 1, the Captain told me it was too soon.
When I caught the GS, I had to extend the gear early because the Captain didn't give me the flaps when I asked, and the aircraft could not decelerate when it caught the GS."
It sounds like your Captain wasn't really doing you a favour. Going down the glide clean is never really recommended in my experience, however, there are one or two people who are happy go down at green dot if intercepting from, say, 17nm or something like you mention. In this case, expect the speed to increase and take gear out of sequence. This is exactly what you did which is fine, but I think the intentions of yours vs. captain should be established early with the brief, so that way you don't end up in a situation where you're wanting something that the other guy doesn't want to give. If you're uncomfortable then say so.
Hope that helps.