I use to get this argument when I worked the brake caliper department at Rebco Racing Enterprises on how closely centered the fixed caliper had to be on an opposing piston caliper. They said it had to be absolutely centered. I don't think it does. They claimed if it wasn't it would put more pressure on the closer side than the other. I thought OMG how dumb are these engineers. Once you put pressure to the system the closer pistons will come out and make contact with the rotor and put no more pressure on the rotor than the pressure needed to overcome the friction of the seals on the opposing pistons until they touch the rotor and then evenly distribute the pressure to both sides. Once the pressure is on the pistons and rotor the system doesn't care where the caliper is. Yes the pistons on one side will protrude more than the other side but seldom can you actually run out of pad enough to pop a piston from it's bore. As long as the caliper is shimmed close to center and misses the spokes you should be good. The purpose of floating rotors is to have lighter rotor carriers so help eliminate expansion rate of the whole rotor during heat up and cool down and has no bearing on whether the caliper is floating or fixed. Keep in mind this offset is only an issue the first time you put pressure to the system. Once you put pressure to the system the 2, 4, or 6 pistons will retract equally centering the rotor.
Not all fixed calipers are wider on the inside, depends on the calipers. My calipers are for a narrow front end and are actually thinner on the inside than the outside and run a shorter piston.