Another question:

Can I do a full installation with admin, database tables, a forum or two, a category or two, and everything else, test it out, and then, if all goes well, do the import from UBB Threads 6.5.5?

I hope you don't mind my asking these questions as I really want to be certain before doing anything.

I'll need to make sure this is clarified in the install or upgrade guide. You don't need to create the admin user, as this will be imported, but you will want to create the database tables and remove the install files/directory.