37gal o.k. for a few F8s?

you could have about 3 or 4 in a 37g, that's it though. No tankmates for pufferfish. You could try, but with 3-4 individual pufferfish that's too much of a gamble. Figure eights aren't generally aggressive, but pufferfish in general are unpredictable and territorial. One minute they can be totally docile, the next they can be terrors of the deep. I wouldn't go with any UGFs in a pufferfish tank, puffers are messy eaters and UGFs are a b*tch to clean. You'd probably do best with an aquaclear 500 and a canister filter. OR you could get a wet dry for that size tank.

I disagree strongly on the RFUG and the puffers, especially Figure 8s and GSPs. I always use RFUG with aragonite substrates for these fish, generally over-engineered RFUG, but simple RFUG is okay as well. It is the best way I know to keep the substrate clean with these messy fish. I use Penguin 1040 or 660(?) powerheads with Penguin reverse flow kits for prefiltration and rinse the prefilters at least weekly.

I would have an additional filter for mechanical only on the tank, as I don't like tanks with no current at all, and the RFUG effectively is super calm water. Besides, extra mechanical is good with puffers.

I don't remember the mid-size powerhead in Penquin's line's numbers (which is why I did the question mark). I use the large and medium from that line.

Your RFUG will carry your bioload, the AC will be your mechanical (supplemented by the RFUG prefilter), so if you rinse at least weekly it is likely to be okay. Puffers are pretty active and very messy, but don't like strong currents much.