hi jem21 - dont think its ever too much. there's just simply so much you can read up on haha. what you have done thus far is good but try to go into the technical stuff like how does the jet engine work, how does an airplane fly, yaw, roll, pitch, etc.
Also try to read up on the destinations that Singapore airlines fly to. Navigation questions seem to be common as well (e.g. where is the Philippines with reference to Singapore)
Once again, try to take it easy and most importantly, be YOURSELF. they value honesty and will not penalize you for wrong answers. more like looking for someone who has the right character and mindset and of course, a passion for aviation
Good luck and all the best!