Registration and billing system

I have a hosting company and I use whmcs and whm for client management and cPanel, which is working fine. I decided to change my front end and use wordpress multisite along with the services from WPMVdev (full membership)

Ok, so I am trying to get the best configuration possible for billing and registeration from start for my multisite.
I have installed pro site to handle upgrades, plugin and themes.
WHMCS integration and WHMCS provision are also installed.
Domain Mapping also installed and I do have an account with eNom.

Questions:
If I use whmcs to handle signup and domain registration. Should I configure it to do the initial free membership only (which last for one month)? and if I do would pro site take over after the one month has ended and handle the upgrade from there?

Answer Form Ignacio

Ignacioat 10:14 am
In any case. WHMCS can setup the sites and you can price it nay way that WHMCS can handle.
Pro-sites is ENTIRELY separate at this point. It won't know anything about a new site unless you program it.
You should probably do the recurring billing in WHMCS for the site rental and pro-sites for the theme plugin etc upgrades.

Nigelat 10:18 am
So in this scenario, the client would have two separate billing system to deal with, correct? since pro site and whmcs or two different systems.

Ignacioat 10:19 am
@Nigel Yes, but I'm not the best to answer about that

Nigelat 10:23 am
If I choose to use Domain Mapping. Does it work alongside pro site or is it also a separate system. and If I did use domain mapping, what would I use for registration?

Ignacioat 10:25 am
@Nigel I'm not completely sure but I think it would be a separate system too.

Ignacioat, this is what I am thinking: I need whmcs to handle the initial free signup and domain registration. Then the client has the option to upgrade at their convince or forced to do so at the end of the one month free period through Pro Site. Therefore Pro Site needs to know about the new site. How is this done?