Hello and welcome @per_coh 
In that scenario, TL1 users would be able to use any pre-existing tag that didn’t have a restriction on it. 
For the second point, as @pfaffman said, there are some Tag options in the category settings where you can add some restrictions to what tags can be used for topics in that category, as well as enforcing that new topics have at least x amount of tags from one or more tag groups:
You can read more about tags in How to create tag structures, tag category restrictions, tag groups and tag relationships