Categorizable is already in some dictionaries, so no problems there.
The problem with commentable is that what you really want to say is comment-on-able, but we can't use -able with phrasal verbs.
So here are your alternatives:
- Just say commentable anyway. If people understand what you mean, then there's no problem.
- Say comment-on-able.
- Find a one word synonym for comment on. I can't think of an exact synonym, but there are plenty of words that might be suitable, depending on context: e.g. discuss(able), criticize (criticizable), append(able), annotate (annotatable), etc.
Which you choose is a matter of personal taste and context. For example, if I was documenting computer software, and if the term comment was already widely used and understood, I would choose 1, because consistent terminology is more important than formal grammatical correctness.