MK5, MK6 and MK7 are all completely different in terms of hardware, none of them are backward compatible as far as game software is concerned, although some games are remade for the newer systems. MK5 has an ARM250 CPU, MK6 has a Hitachi SH4, and MK7 is basically a PC running Linux. The game chips are also physically incompatible - MK5 has 40-pin 27C4002 or 27C4096 EPROMs, MK6 has 42-pin M27V322 EPROMs (MK6 base and set chips are different again, being smaller variants e.g. M27V160), and MK7 uses CF/CFast cards with a SIM card for good measure. Cabinets on the other hand can be upgraded to a later machine, for a MK6 you need to make sure you have a second screen if it's required by the game; MK7 always needs two screens. I have even seen a widescreen LCD shoehorned into a MK5 MVP cabinet (the game was a MK6 Black Rhino Power Pay) so I guess anything's possible (although Viridian WS/MK8 games might not like the fact the buttons aren't OLED). I have no idea if MK7 and MK8 are compatible with one another.
A complete game list would probably be upwards of 1000 titles, although not all games were released in every region.