What brand are the servos and Rx? Chances are they would all handle 6 volts with no trouble. I switched to all 6 volt Rx packs back in 1980. Never a problem with any of the systems I've used since then. But even a 4.8 is fine. The added MAh will simply allow you to fly longer before having to recharge. However, make a habit of topping off the battery before each day of flying. You always want to start with a full tank.

Stick with the established brands- Futaba, Hitec, JR, Savox, Spektrum. There are people using some of the newer, cheaper brands like Turnigy and Solar with good success, but the cost savings is pretty minimal compared to the risk IMO.

Alas, JR is/was a good brand until recently when they went under. However, I've heard rumors of them coming back. Bottom line is they're in flux as a company right now so I'd use what I have but at the moment I wouldn't purchase anything new from JR.

Mixing equipment brands is OK as long as you know what you're doing. Any Receiver will NOT talk to any Transmitter. Their modulation schemes must match. Terms like DSM, DSMX, FASST and others are the mod schemes I'm talking about. A rule of thumb would be to pick one of the schemes and stick with it. Usually whatever dominates at your local club field.

Turnigy came out several years ago as an inexpensive alternative to JR, Spektrum and I even think Futaba. I've never used one and can't comment on their quality. But my impression is they're a second tier brand of radio. i.e. something to buy when you can't afford better. Not bad but not the best.

All servos are pretty much interchangeable brand wise. You have to pay some attention to voltages and wire order and such but in general, it's no big deal. However, as mentioned quality can vary. You pretty much get what you pay for! So you may want to avoid the $9.98 jobs.

One thing I will do is keep all servos in a given model the same brand. I do not mix brands of servos in a given model. There were some issues many years ago (1980) where a certain brand of servo could not be used with other brands as it would "Sometimes" cause problems. Maybe! I don't think it was ever really determined what caused the issues but I just made a personal rule back then to keep servo brands matched in a given model.

One place I like for info if nothing else is Servo City. Link You can research the specs on a lot of servos here. They push Hitec which I've found to be a decent servo for most applications. But like everything, some people don't like them. And like everything, you can get a bad product now and then. Always test your servos in a model you can afford to lose. A big/expensive model I would go with something else however.

Another couple of good servo links: Servo Database

And: Servo Wiring