are rotors a standard width? can they be used with different brand of brakes, ie hope with avid?

If you are talking about the thickness of the rotor then I would say generally speaking – yes. Of course the rotor diameter must be correct for the caliper/adaptor combination. You may get clearence issues with some combos eg. saint m810 with Hope floaters on some frames/forks. Best to speak to mechanic who has come across more combinations than I have.

ive swapped over loads and never had any problems..

Same, although floating rotors present problems with hayes systems as the spider of the rotor can make contact with the tabs on the pads, but in this case, on the girlfriends bike i just snapped the tab in question off and job done, need pliars to remove the pad now but aint no biggie.

Use Hope Floaters with XT calipers, had to file some of the caliper mount away where the bolt goes through to the adapter cause the rivets of the flaoter caught as they went by. No worries once done though and no hassle doing it either (used an angle grinder VERY carefully) and job's a good'un !