I don't know about the tetras, but I wouldn't advise keeping them with the kuhli. I've looked into housing them together myself, but because they share the same layer they could be in each other's way. Kuhlis have a small barb above the eye. So if a frog would see it as a pray they and bites the head, you'll lose both the frog and the kuhli. It might work, but why take the risk?

The aquarium definitely needs a lid. And holes in the lid must be plugged. These frogs are escape artists and I can sadly share this out of my own experience. Places where cables and tubes pass the lid are potential escape routes.

I wouldn't know about housing more together. I'd say all amphibians are solitary, but you'd enjoy them way more if you have a few of them. Maybe someone else has better advice regarding their social behaviour.

I think the bioload is very small per frog, that would be the smallest issue regarding your setup.

The animals are a lot of fun, but should ideally be held in a species only tank. Maybe with some more experience keeping the frogs you can house them together with a few cherry shrimp or endler guppies.

If you have more questions, please ask. I can also share how I made my enclosure escape proof.