My F8 jump like little so and so's, so a bad idea for an open tank, they'll be up and gone! Ideally you have 15g for 1 F8 and 10g for any others, so you may be OK, I would use that size tank but I'm sure others would disagree with me!
My F8's eat snails 4 times a week as it's important to wear their beak down, have you considered asking the LFS for some once a week? Avoid Malaysian Trumpet Snails tho as they are very hard and can break the puffers beak. Here's what mine it: Pond/Ramshorn snails, opened cockles and mussels (they won't get them open on their own) blood worm (their favourite) tubiflex, krill, brine shrimp, prawns.
They require very precise water parameters, and need very regular water changes as the nitrate in their tank can sky rocket quickly because of the type of food they eat.
Did you realise they are brackish fish? They will tolerate freshwater for a few months but ultimately need brackish with an SG of between 1.005 and 1.008
They don't mix well with other fish although some people say they do. I considered bumble bee gobies for my tank but decided against it as they are still juveniles and quite placid but may eat the BB's as they get older and more aggressive and the tank you've got probably isn't big enough.
I keep puffs and my daughter has guppies, which are a million times easier and I've yet to see one of her beautiful boys jump, so they may be the better option for an open tank.
Hope this helps and good luck, let us know what you decide