For question 1- They should all be fine. If you have any problem, it might be due to the spring since they are used? But if you are concerned, you can buy and replaced the springs and follower for them. I've seen them in 3 packs if I remember correctly? In fact, here you go...
http://www.brownells.com/.aspx/sid=1...air_Kit,_3-Pak
For question #2- Obviously the Colt brand will be fine. Never heard of Kay? And since you can't remember the other brad/s, can't really help you there. If they're all made to military specs. they should all be fine? Even though made by different manufacturers, the military specs. are still the same.
As for question #3- I wouldn't paint them personally as the paint might make them stick or hang up or not work well? They might get stuck or not fit properly in the mag well? But, I suppose you could paint them if you really wanted to, but I would ONLY give them a few really light coats, ONLY enough to cover them. And even then, you might have to lightly sand or you might have to remove some paint where it goes into the mag well?
Lastly and again, if you have any concerns, dry run all the mags you have with a full load of 30rds. If they work manually, the odds are they'll work under fire. But again, if you have ANY concerns, just get some new silicone springs and green followers for all. Not all that expensive.