figure of 8 puffers get to about 3" (excluding the caudal fin), they are one of the smaller species. Green spot puffers are a very good choice for brackish tanks, they are slightly larger species.
Puffers can be kept together if they have enough space. I actually keep a couple of young green spots together, though their aggression sometime comes out when older. However, if raised together the aggression towards eachother has been recorded as being lessened. Figure of 8's are easier to keep with other species, but are still difficult, they will still fin nip, and will happily pick on species much larger than themselves. Green spots are even more notorious.
i think if you are careful with the stock you have a good chance. That is a reasonably large tank, you may not need the dividers Puffers are very inidviual, so you may get very clam ones. Like i said, i have a few in a tank together and they get along fine, especially as i've has them together since they were very young (as most shop stock will be).
yes, i see no reason why not. The crabs are most likely a species of fiddler crab (most commonly know as rainbow crabs in the aquarium trade) and having an area for them to spend some time on land is essential for their well-being. Often this is done just having a sloping sand bank or similar on one side of the tank, or by having some large branches coming out of the water.
Mind you, it must have been a good sized tank! Archers are quite large, and mono's can be too, though they are moderate sized brackish fish the archers, if healthy, can get to some 10 inches! Sounds like a good set-up though - i have seen similar run successfully in public aquaria. I guess my only worry for the crabs might be the puffer, if it got hungry it might try and have a go!! Mind you again that would depend on the species of puffer, the size of the crab, and whether the puffer was one of the more intellegent of its kind Having something cave-like is great for puffers, mine like to lurk in the holes in my rock formation (i say lurk, sleep would probably be more fitting.....)