Obviously there's some safety issues running an unguarded grinding wheel. Apart from the obvious ones of directing sparks and debris, and keeping you/stuff from touching the wheel where you/it shouldn't, the guard protects you somewhat from flying bits of grinding wheel if the wheel "blows up".
Ignoring that for a minute, the other issues of putting 8" wheels on a 6" motor relate to torque, and RPM. Putting a bigger grinding wheels on makes the motor work harder. If this is a "big ole Baldor", it can probably take it, but if it's an import... You may have to press the work into the wheel "less hard" to avoid stalling the motor. You may burn out the motor or wear out the bearings sooner than you would with the smaller wheels.
The real big issue is RPM. Make sure your 8" wheels are rated for the RPM of your 6" machine. If you run the big grinding wheels too fast, they can "blow up" even if they are not defective.
It's pretty rare to get defective wheels these days, but it's pretty easy to damage a grinding wheel by bumping or dropping it (or if UPS dropped the box on it's way to you leaving a little internal crack) So I would encourage you to figure out some sort of guard.
The arbor size isn't really much of an issue, as long as the hole starts out big enough. You can get sets of plastic bushings at good industrial hardware stores that will take care of just about any possibility.
