How to...Free trial...

Free trials are simple to implement.

You insert into the users table their join date. You log them in and have a session with that same join date. Extend the controller to see whether the date is greater than 30 days. If not, proceed as normal. If so, then display a trial expired page.

I wouldn't setup subdomains, keep it simple like http://www.domain.com/clientA for example.

Each client doesnt need its own database. You can have 1 database, with a clients table and then have a client_id column in the other tables.

This is pretty simple stuff.