Welcome! I am guessing that is a Pea Puffer? A common tank mate would be a Kuili Loach. Cool fish and usually out of the puffers way. Neon Tetras should also work as they are quick and the puffer won’t be able to catch them. You could also try a small catfish. Cory Dory’s are quick and might work but they are also kinda the same shape as a puffer so might attract attention. I haven’t had a puffer & this is just experience from discussion with a friend who did.
I am not familiar with the upside down at all. I would be researching and watching video on it.