Yes, CAC can limit the number of calls within a Mitel cluster. You'll have divide your sites in zones and ZAP which are your MPLS links between zones. Zones and ZAP must match your network topology.
From my experience, that's not so easy to do.
All this programming is done with Network Zones and Bandwidth Management forms. Also, online help has great examples.
Finally, you must know that a Mitel 3300 cannot do G.729 on its own. You must buy a DSP II card and appropriate licences. A G.729 resource will be used when the 3300 controller is involded in the path (like a phone call between an IP phone and a TDM trunk/phone connected to the 3300). Phones are able to do G.729 for P2P calls. G.711 will be used if a G.729 resource/license is unavailable.
With Network Zones form, you'll be able to define which zones have to use G.729.
Also, note that some Mitel phone models can use the G.722 codec which uses less bandwidth then G.711 (even if the voice quality is better).
Hope that helps!