The miller is the primary focus and will be built with no compromise in terms of speed/strenght/rigidness.
If you then *have* the milling gantry, why not just use it even if it is overkill compared to what you need?
Is it not just a case of getting the cutting head around at the right speed?

Take florins' design for instance (http://www.mycncuk.com/forums/showth...ll=1#post13298) Why cant you use a cutter on this design?
Of course I know that you are going to burn the bed away, thats obvious, so you will have to swap it out if you are cutting metal.

BTW: love your machine florins