Fitting GS handguards onto a Rockster can be done. For best results, get your hands on a Dremel tool as the cable cutout on the right guard needs to be extended. Also, they don't sit square on the bar brackets so I rounded the corners off for better fit.
I have both the GS handguards and Throttlemeister on my Rockster, with no problems. They work GREAT together! Bought the guards online from Max BMW, and did have to do some dremel work on the left guard to allow for choke and clutch cables. The bike came with a Throttlemeister, but I had to order the GS end piece directly from Throttlemeister to accommodate the handguards. I'd say go ahead and order the whole thing directly from them. They are helpful and very knowledgeable about applications. Their website is http://www.throttlemeister.com.
Citrusbbee
Member #553
'04 Rockster Ed80 (#967)
'15 R1200RT