I've kept F8's and GSP's together in the same tank with success. Mind you, they were juveniles. My GSP moved into another tank because of his adult size being nearly twice as the size of the F8 and his salinity needs.
I would start with 2 F8's (maybeeee 3) and one GSP. That's just me though. If you go for 3 F8's I definitely wouldn't go for 3 GSP as well.
I did have to play a few tricks on them though. Hiding places, including caves, heavy planting, and driftwood are excellent ways to break the tank into "territories..."
I also fed them at seperate ends of the tank at the same time (you grow accustomed to multitasking with them
) that way they both had a good chance of eating with little to no stress. Of course, I never had a problem with them attacking each other or nipping. The only time they showed a nasty side was when they were stalking ghost shrimp, fiddler crabs or crayfish.
If you're looking at keeping them together for about a year, you shouldn't run into a problem. You should buy some Knight Gobies and perhaps a few Bumble Bees! I really need to upgrade my 20g to a 55g at least. So many more possibilities!
Best of luck!