Correct, the Sam Sho 2 cab pretty much only works with Sam Sho 1 and 2, button wise. I DID buy one of those 108 in 1 carts, and would you believe that Samurai Shodown 2, on that cart, really isn't very good. The sound is really off. Too bad.
Yes, I have room for one more... and I'd like to get a Street Fighter in there (again). I got rid of the Third Strike, as it made me nervous about dying. Plus, the cab that one was in was slightly poor compared to the rest of my collection. So, I got rid of it just because I didn't want a sub par looking cab in my collection. If they crack that stupid CPS3 code, I'll get another.
STTNG cost me 2400, then I added some more shit to it. A bit pricey for Star Treks, but it is in super condition, so... They can be had (supposedly) for 1900. But, I'm sure they'll be junk. People always say you can buy them in the teens, but I've never seen a good one go for that little.