Figure 8 Puffer

[IronManatee54]

I am planning on getting either 1 or multiple figure 8 puffers. I'm not sure if I have room for a big enough tank for multiple figure 8 puffers. I'm not sure on the exact size of the tank I have currently but it is about 20" long, 10.5" tall, and 10" wide. I had a figure 8 puffer in it before but it unfortunately died. I currently have 3 GSPs and 1 Northern Puffer. I typically just feed a random clam that I find in the Northern Puffers tank. Ill always just crack open the shell and then hold it in the tank for a little. Is there any way I should be doing this differently? Some other questions that I have:

Can I feed the figure 8 puffer a clam as well? Or should I get snails? If so which ones and could I just leave them In the tank or would it be better to keep them in a seperate tank?

If I were to get multiple how big should the tank be? I'm pretty sure I saw somewhere that it should be 15g for one and then +10g for each puffer.

What should the decorations be like? The current tank that I have just has some regular aquarium rocks and some decorations. Do I need to have sand and plants with lots of places for them to hide?

Do they do better in a community or single species tank? If it is a community tank I was thinking of getting some Bumblebee Gobys. What would be the best fish to have in it?

Lastly what should I do for filtration? I currently just have a sponge filter. Is that okay?

I think I covered All my questions if there is anything else I need to know about figure 8 puffers or the other 4 that I have anything would be greatly appreciated.