I would like to build a set up similar to the one used by trulia.com QA section:
Where the user can select both a category and a location for his question.
(Location being optional)
I was thinking to utilize the tags functionality to simulate the location field.
So my question is if
a) there a way to force the user to only use maximum one tag?
b) there a way to only allow users to choose tags from an admin-specified set of valid tags and not being able to define their own tags?
Any hints on whether the above are possible without customizations or any hints on how to go about hacking the code would be more than welcome!