I currently have:
Guru Mighty - plan on starting the THS with this. Then switch to the Rack Attack Grape Solid when the lanes start to dictate.

OK - here is my situation:
Should I get the Primo to put in between these or get the Guru Master for when the conditions are slicker?
Most of the time, these will be used on THS leagues.

Guru master is more for your long patterns with heavier volume tournament or PBA league patterns.  Easily the Primo here for a THS league.  If you went with the Mighty, Primo, and Grape Solid you've got a pretty good arsenal for league.  Put a shine on Mighty, a 4000 on the Primo and hit the Grape with something like 1000-2000 for nights that play a bit tighter and go.....