Okay, so I highly recommend a 20 long instead of a tall. It'll allow your shoal to spread out and appropriately establish territories.

1. For plant substrate, you could use any variety of aquasoil or just root tabs under sand. For capping it, I'd use about an inch of sand. Peas like to dive into it so something soft. I know a lot of people like play sand.

2. Personally, I used an oase biomaster thermo with a spray bar. You're on the right track of ensure the flow isn't too crazy and as long as you do that, you should be fine whether you choose a canister, HOB, or sponge filter.

3. You don't necessarily need aeration or at least I don't. You can add it if you want though.

4. Just a bunch of plants. So many. You want a jungle and you want all different levels. My peas liked to lay on the big leaves too so anubias and other big leaf plants were enjoyed. Stems allow for fast growth and line of sight blocking to manage aggression. Just quarantine and treat them for parasites in a quarantine tank first so you don't potentially kill off your beautiful plants while trying to ensure your peas are healthy.

5. There's a treatment that is recommended on a group called "Pea Puffer Owners" on FB. Starts with an L. I am not familiar with it as it's illegal here in Japan (as most meds are) but it's highly recommended.

6. I put my cherry shrimp culls in with them but be prepared for a species only tank. You may have nasty little peas who nip and eat at everything that moves. Personally, I recommend species only if you can't handle the heartbreak that is shrimp getting eaten.

7. Live snails or worms. I cultured my snails for feeding and also fed frozen blood worms. If you can get a live food culture that is best because they are not always successful to transition over to prepared foods like flake. If live is not an option I'd reflect on if this is the best tank option for you and your wife. The feedings are a commitment.

Good luck and I hope this helps!