Hey DT! Yes, I'm still torn on inhabitants, I'm having a real mind boggling research adventure on the whole thing. I have kept F8s before, but not a GSP. The initial plan for the 36G was to keep a GSP on its own. But there's the option of testing hermit crabs with that, since they're both full marine (well the GSP would be eventually), but by 'full marine' we're talking 1.015 really, which is below seawater (which is about 1.025+, I think). So it'd still be lower than normal for the hermits.
The initial worry would be that the GSP would eat them, yes. But, pufferpunk over at the Puffer Forum has kept hermits with an adult GSP before for around a year, by providing lots of shells. It also depends on the puffer's personality. This is the problem with keeping puffers in general!!! Some people manage to keep BBGs and mollies with them, while other people have had problems. I suppose I won't know unless I try...
Another option is to keep two F8s, and possibly go with a pair of orange chromide cichlids as well. This setup would remain low end brackish. Though, I now read conflicting advice about F8s being freshwater in fact (fishbase.org specifically states "not a brackish species").
Now, in this setup the S.G. would be around 1.005, which is very weak brackish. So, I have the "hermit dilemma" of: GSP (more aggressive, more likely to eat them, but same S.G. or thereabouts) vs. F8 (less aggressive, probably won't touch them, but far lower S.G.).
And, I know with inverts it's extremely difficult if not impossible to acclimatise them to conditions any different to their usual habitat, to top it off!
I really need something to till the sand that's going to be in the tank either way. Snails are likely to be eaten if they're small enough, though my previous F8 used to leave larger ones alone. So, snails are out. I can't use MTS because they can break a puffer's beak. Kuhli loaches would make a nice snack I bet, so they're out too. So, that's why my thinking led
me down the path of the hermits!
If hermits can be adapted (:S almost certainly not, I know) to a lower S.G. then with F8s I doubt there'd be any problems. It's a question of salinity though. With a GSP it'd be a question of ensuring the hermits could remain undetected for as long as possible, introducing them with a very young puffer, and hoping that the puffer's personality is less aggressive than normal. Bit more risky.
All in all the best option is probably not to go with hermits! But, then my question is: what else is there to till the sand?!