How do I let people choose from a limited set of tags when they post? In other words, if min trust to create tag is level 2 and min trust level to tag topics is level 1, how do level 1 users choose a tag for their new topic?
Also, is it possible to force users to choose a tag?
You might be able to achieve what you want with Tag Groups. Go to Tags and Manage Tag Groups. You can set who can use what Tags by User Group by Tag Group. You will need to add the relevant tags to the relevant Tag Group
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: