This is a short question: Is there any limit for the "tech levels" in the AI files, or I can define as many levels as I want?
There is no limit i'm aware of.
Having said that, I've been slowly coming to the opinion that most of the things we use tech levels for are better done by keeping the build repeat figures low to force the AI to progress up the build list.
The real problem I see a lot of the time is that AI's are trying to be to elegant and are getting caught up in infinite tech tree loops as a result. Personally, i'd follow the KISS principle, it works as well with AI's as with anything else. =p