For lighting a backdrop is it best to fire at full power? Let's say I had a blue gel over the flash, is full power still best? Or does it depend on what result I want?
Yes, it depends entirely on the result you want, but in a studio situation you are often shooting at the higher end of the power output range. The biggest problem with hot shoe guns in a studio portrait situation is that they tend to be working very hard, if not flat out, and recycle times get long. (Moving the flash closer not only softens the light with a softbox/brolly, but also increases the brightness a lot so you can turn them down a bit more.)
Anything more than two or three seconds is a real pain and the batteries soon tire. If I had a pound for every picture I've taken where one or more of the flashes didn't fire, or went off half cock, I'd be very happy. Studio flashes recyle in 1-2 secs max. If you can reduce the power of hot shoe guns just by half recyle times are roughly halved.
do you think i should buy a high end flashgun (e.g. 580exII), then a few other cheaper / 3rd party flashes?
You can do a lot with just one powerful flash and the priority should always be to get the main key light right, with enough power to drive a softbox or brolly.
All the others are secondary fill lights (use a reflector instead) or effect/accent lights or background lights. If it was my choice, I would start with one good big unit and look to introduce second and third units as and when. This would also be a good learning experience too.
The new Nissin guns are getting good reviews and are usefully cheaper than Canon. Nissin are a big company and have always been good quality - they manufacture stuff for camera makers. Sigma are also worth a look.
However, if the stuff you want to shoot now demands two or three lights, then you havn't got much choice.
