Do you think I should retire the old templates?
You could keep all of your current default templates, as miniBB should work with them (and it worked).
BUT, if you install the Mobile add-on, then you should add to its own set of templates the new versions. By default, it uses some core templates to avoid duplicates, but in case for older templates, these things are incompatible.
I think it should not be difficult and long to arrange all templates you have with common ones, so feel free to contact me if you get lost in them.