You can do a lot with an XP-6. 6 addressable IR ports each of which can drive 4+ emitters, plus two dedicated serial ports, plus IP control capability.

6 screens sounds like 6 zones two me, which matches up with 6 mpio ports. The source equipment is minimal. the switcher is serial, looks to have IP but no driver that I know of. In fact why is this switcher being used at all other than getting something that is cheep? Which makes me think that the budget is very tight which is why the installer is going with the XP6 over the 8s.

The XP6 can certainly do the Job but it doesn't leave a lot of room for growth other than through expansion modules.

Bottom line, if you have the money get an XP8s, if you don't go with the XP6.
however, get a real matrix switch like a zektor, Snap AV or anything else that has a decent track record and a functional control driver.