That is a poor combination for a couple of reasons Im afraid, it might work out as they are juvis but there would be definate aggression between them all as they grow, it is hard enough to ensure own species get along alright as they get older. It also is a bad combo as the figure 8's are at the low end brackish and the GSPs go much higher end, so high some peeps take the tank into full marine for easier maintenance.
The tank itself is a nice one (just googled

) Puffers generally prefer length over height but I have a puff myself in a cube tank which he enjoys (I put it down to the landscaping

). Anyways here is a small low down on the two.
F8s - 15g minimum for one and 10g per extra (though 15g each best bet). People have successfully kept them together and also with tankmates such as knight gobies and bumblebee gobies, the two I had lived very peacefully with a silver molly (again wasnt sure how that wouldve panned into adulthood). General water params:
Low-end brackish water (SG = 1.005 - 1.008); Temp:24-28C (76-84F) ; pH: 7.8-8.3
Get to 3 inches
GSPs - 30g minimum for one (and would say 30g per puff). Again there are success rates if grown from juvie that they can live together, some have even had one in a full marine setup with tank mates but there are inevitable losses. Puffers have individual personalities so it would touch and go if it works. General water params:
High brackish to marine salinity; Temp: 2326C (73 - 80F ); pH: 7.0 - 8.3
Get to around 6 - 6.5 inches.
Hope that helps a little! TO broaden horizons if set on puffs, others you could try are:
Red Eyed Puffs - they work well together and could get 4 in there (perhaps more)
Dwarf Puffs - 5g per puff )some work 5g for first and 3g for each additional) they are full of personality but it depends if wish for a bigger one
Congo Puff - gorgeous colours such as red or orange. Could have one but is a lurker and buries.
Abei - again one, I have one of these extremely active little fellow.
Hairy Puffer - unusual puff not so common, again one of these, though have read peeps have breeding pairs.
Brown Puffer - again one and simialr to abei
Suvatti - a funny looking puff and similar to congo, a lurker.
Dragon Puff - if went a little bigger to 150 litres then this would be a possibility too, these have successfully been mixed with suvatti (1 palem, 3 suvatti) but only in big enough tank
All the alternatives above are freshwater, sorry to babble!
EDIT P.s. other folks will come along with their experiences!