[Zulithe]

Bomys have more HP than humans with more defense than humans
Zulites could have more HP than humans yet less than Bomys but more defense than a bomy. Or they could just have the same defense/hp ratio, either way they would be balanced.

Now while a Zulite is physically stronger than Humans and Bomys, they are also slower than both Humans and Bomys which will make it more difficult to hit either of them. (not taking into account that Bomys are faster in water and Zulites are faster on land while rolling which balances out the fact that Humans are faster while walking than either of them)

So I think that they are pretty balanced as it is.