You can connect third party sets but you need the proper licenses installed. If you look in the BCM (Release 6.0 anyway)under Resources>Application Resources you will see parameters for Avaya SIP phones, IP sets and Other SIP sets.
Cisco sets will work but like all the third party sets, they're a pain in the butt to set up and they don't work nealy as well as the Unistim sets. Most SIP sets will work but my opinion is they aren't worth the effort. Some of them may be cheaper to buy but you will pay a higher price to get the buggers working so in the end you don't really save much.
As far as I'm concerned, if you want to use multi manufacturer sets on a single system, your best bet is to replace the BCM50 with an EMetroTel UCx system.