have seen lots of software for 3650 and 7650 phones but little for the 6600....
because the 6600 is a series 60 platform should the software work regardless of the model?

Most apps work OK. Some might not (apps using undocumented functions that no longer work on the newer Series 60 release, or work differently), for those you often can find a newer version that also works on a 6600.