I agree with what the others have said: GSPs grow way too big even for a 30G IMHO
. An F8 would be ok, but they need very low end brackish (1.005 max S.G. which really on the face of it is barely saltier than freshwater). I know people who keep BBGs in with F8s but its a risk you would be taking: they may end up food for the puffer.
Do you know that mangroves are 'tank busters' and require an open top tank? Puffers are notorious for jumping, so I'd either go with mangroves or with the puffer but not both, but personally I know of too many problems associated with mangroves to risk using them.
If this is your first time at creating a brackish environment, and you only have the 20G to hand, you may just want to go for simplicity and keep a single F8 in there
Trust me, it will be just as enjoyable
They're great little things.