I don't own a TS 75, so maybe someone can confirm the bit below that's specific to the 75.
With the TS 55, the 1400 rail will crosscut a piece of 4x8 plywood, but it's really close. I've generally been ok doing so, but I sort of wish I had the 1900 instead. Ditto the 2700 rail and an 8' rip. If I had it to do over again, I would likely get the 3000.
However, my understanding of the 75 is that the saw's base plate is larger, and those close calls with the clearance on the 55 are even tighter.
I purchased my rails in a regrettable order: 1400 (with the saw), 2700 (when I wanted more capacity), 1400 holey (when I got LR32), and then 800 (when I wanted something small). If I had it to do over again, I definitely wouldn't own the 1400 non-holey or the 2700. I would start with a 1400 holey or a 1900, and if I were going to have a long rail, it would be a 3000 (I do like having the long rail, but it's not essential, if you don't mind joining them together). I still like my 800 for little things.
There's also a terrific thread on here that discusses cutting rails to custom lengths. If I were starting over, I would seriously consider going that route.
EDIT: Garry beat me to it, but that's the issue I was referring to about the size of the TS 75's base plate.
Also,
here is the discussion of cutting rails to non-standard lengths.