Hi gmat730must,
To start, while the GMAC CATs are the most realistic ones on the market, the CATs offered by
MGMAT, Kaplan and Veritas are 'close enough' to the real thing that they will provide a reasonably accurate assessment of your skills (when used correctly). As such, you have more than those 2 GMAC CATs that you could use.
As far as when to take your CATs - you're in the final month of your study schedule, so taking a FULL-LENGTH CAT (with the Essay and IR sections) under test-like conditions each week would be a good idea (and your last CAT should be about a week before your Official GMAT).
GMAT assassins aren't born, they're made,
Rich