will it take a lipo rx pack 7.4 and will the on a off switch be fine with it too

The SR300 will take a 7.4 LiPo pack but your servos won't unless you're using high voltage servos. The reason you see electric vehicles using a 7.4 without a BEC is because it's built into the speed controller only giving the receiver a voltage of @5/6 volts.

the servos i like are
5585
5565

You are good to go. I think those are actually good budget HV servos for brand new. If they are new. I wanted a pair for back up in case my Savox blows. Good luck.

will the on and off swithc be fine to