Is something like a tool building system in-game as the main form of combat for a rougelite, platformer, fps also allowed

How I would think of it is if the tool building aspect is 50% or more of the game, then yes. It should be the main/central gimmick. I guess something like MOTHERGUNSHIP's gun building system would be a good example of this:

Thanks for the clarification also, ironically that game was what I first thought of when I read the rules