The type "A", "B" or "C" just refers to the offsets. The actual offsets they represent vary depending on the width of the wheels, but A is always the most negative and C the most positive variants of offset. Here's a handy little chart I made that represents this for most of the older SSR wheels.
Regarding the wheels, the centres should have a ~1mm clearance around the edge of the centre when mounted normally (as you've probably noticed) so you shouldn't have to force them at all. Reverse mesh centres are indeed larger than the Formula mesh centres. There shouldn't be any difference from two different wheels where one is a high offset for FWD applications and the other is low offset unless they differ in being Formula/Reverse.
That's good to know, but what wheels and size is that for? My 14" Reverse mesh are 9j-16, and my XR4s are 9j-25. I'm not sure what type either of them are, but the numbers are a bit off from your chart. As well as the formula mesh I just took apart. The type A is a higher offset than the type C.
Also, there is a difference between type A and C that I just found out. I'll put pics up later, but
The faces are different sizes.
I have two wheels I checked against. Both are Formula Mesh
15x6.5 +10 type C. The face is 337mm in diameter
15x6.5 +17 type A. The face is 320mm in diameter
The reverse mesh I have, the faces are 343mm. I know we've discussed these are bigger, I just wanted to add the size for future reference.