Richard,
Seems like you've just started with marketo. Congrats!
I'm not sure that you need to rush into the RCM/RCA system if you don't have your lead lifecycle nailed down. Why not spend the time to make sure the plan is right and the other parts of Marketo work, then work on the RCM implementation?
That being said, yes you can start up an RCM at any time. Marketo usually recommends you Edit a running Model rather than deleting it because if you remove an existing Model, that data is lost. Of course if you start adding Stages and routing rules in the middle, you'll need to rebucket alot of leads by hand, which might impact your conversion ratios.