Adding isolated exchange 2010 CAS and DAG without affecting Production

We currently have a single CAS server (exchange 2010) and one back-end mailbox server (exchange 2010). I wanted to start with a clean slate and create a CAS cluster (2 front end) and DAG containing 5 back-end mailbox servers. I need for this new implementation NOT to impact my existing environment. What is the best method of moving forward with my implementation? I have read some threads stating that after you add a new CAS server, mailboxes may be redirected to use that server instead (which would cause problems in my instance because i don't want this to be in production until i have thoroughly tested the new setup). After installing all roles and verifying setup is how i would like it, how do i automatically cut over clients to use the new CAS array?

Also had questions related to best practice of creating databases. We have 1500 users, and am unsure as to what the best way of moving forward would be in order to obtain high availability (on DAG) yet remaining conscious of storage consumption?