Well, disregarding my dislike for relying on the use of fans, those 12v fans might run a tad slow on the receiver voltage of 5 or 6v. If you are running 2s lipo (or 6 cells), they should work, but have less output than they would at the full 12v. A 12v fan might not be able to start turning with a low voltage like 5 or 6v, so running them straight to your 7.2v battery pack would be recommended.
There are 5v fans around, but they are harder to find, but can be run straight from your receiver provided you only use one or two.
Yes, the higher the amp rating, the more it will pull from the batteries, but those numbers are all less than 1A, so any decrease in runtime will be negligible. However, if you run those at less voltage, the current draw will be less as well.
THanks. That makes sense they would run slower on less voltage then 12v. So with that kind of amps it will not be noticable. I did order a couple of 5v from this link the otherday. It's nice to have the added security of a fan. I try to do it all by gearing. Had an extra novak fan and put it on my mamba esc and it was just nice to check the temp and it barely over 100.