bbellfly...

I use Futaba radios and mostly Hitec servos. I like the Hitec. They are less money than Futaba, fully compatable and I've had very good luck with them.

1. Yes.
2. Your control surfaces won't "wash back" and will hold better, faster response (very important when doing hovers etc.) better centering, more torque, have another place to spend your money
3. I use them in all my airplanes including the 40 size. Some of them I upgrade to analog servos (Hitec HS645 for example) which is way better than the 3004 and yet is not digital. I use them on the ailerons only
4. As I said I would use the Hitecs. Ailerons = HS645 Elevator = HS5625 Rudder = HS5645. These servos are kinda of low end digital yet work very well. On larger planes I step up to the HS59XX servos. They are better in every respect and are more expensive.
5. No. Great place to use up the 3004

Thanks
Barry