If I was to install a CM5.0 or 6.0 system into an exiting enviroment where there are 7911, 7912 and 7940 phones running SCCP can these phones co exist with the newer 7900 phones that support SIP. I guess im asking if i can have a SCCP and SIP mixed enviroment.
SCCP and SIP can co-exist without any issues :) Here is a clip from a good White Paper;
Enterprises with a Cisco Unified CallManager can use Cisco Unified IP phones with either SIP or the company-developed Skinny Call Control Protocol (SCCP). These protocols coexist within the Cisco Unified CallManager environment, allowing gradual migration to SIP while protecting investments in existing devices.
From this doc;
Hope this helps!