"There are also seven cats in the tree."
I'm not a native Chinese speaker, but I've never seen/heard the word 也 in any position other than right before the verb. For example, I've heard sentences like 我也有一只狗, but never sentences like 我有一只狗也 or 也我有一只狗. I'm almost certain those latter two sentences are ungrammatical.
Both are fine. There is one subtle distiction though: 树上还有七只猫 would definitely mean "there is something else kn the tree, but besides that, there are also seven cats." If you use 也 on the other hand, it could mean that, or ut could mean that there are cats at some other place, but there are also seven on the tree.