Typically for a 35% scale aerobatic plane, you're going to need one per elevator half, two on each aileron, two on the rudder.

If you plan on doing 3D or competing in IMAC, you're going to want GOOD servos, like JR 8611A's, Futaba 9152's (I am not too familiar with Futaba, so forgive me if I got the number wrong), or Hitec 5995's. Servos like 8411's or 5945's will work also, but you might not have enough torque with only two of those on rudder.

I agree that you will need not just good batteries, but also good regulators if you are going with a 6v setup. If you are over-amping the system, you will notice it in the air. If you want to go with Lithium-ion batteries and regulators for them, Fromeco or Duralite are some of the best in my book; I have no complaints with either.

To match the servos together, you can use Matchboxes from JR or Equalizers from Smart-fly, which will work on most setups.

Hope this helps,
Brandon