Question 1:
Yes just for resolution support... Callisto is an IOFramebuffer, not an IOAccelerator.
You can use an IOAccelerator modded for using Callisto as the framebufer to attach to but that doesn't work for all ATI cards...

Question 2:
Just edit in CallistoHAL.kext the Info.plist
Modify IOPCIMatch to match your videocard.
Standard it is set to 5a621002&0xffffffff (ATI Radeon Xpress 200M)
Thus first the device id and then the vendor id.