On the Mac, what if Disk Utility is used to mirror two hard drives, one is 1.5TB, and the other is 2TB? If I happen to have two external hard drives, one 1.5TB and then other 2TB, and use Disk Utility to mirror the two drives, will there be any problem?

Can it be mirroring just for the 1.5TB portion, while the remaining 500GB is for storing un-mirrored files?   (but will Mac OS X always try to mirror those files and try to copy into the 1.5TB drive?)

(will it be the same if the two drives are 2TB and 3TB, or if the drives are different brands, so the capacity might differ slightly?)
In this situation, I recommend using disk utility to partition the larger drive such that one partition matches the size of the smaller drive.  Then setup the partitions of the same size as a RAID1.  You will end up with a mirrored partition of 1.5TB and another un-mirrored partition of 0.5TB based on your first example.

Same thing will work for a 2TB + 3TB drive, but the mirrored/un-mirrored partitions will be 2TB and 1TB, respectively. Brand won't matter and you can use disk utility to find the exact sizes.