Single cell can be mixed at club races for now but things will change as we figure out the cars. What is "equivalent" or "close" now will not be the same tomorrow.
17.5 has no "equivalent" to 4 cell and it shouldn't. Personally, I think that this is the class that most club level racers should be in along with any new racers. Slower stock speeds will bring back the oldschool snake of cars running around the track. If you missed stock racing in the midwest back when the cars were slow you missed a lot.
13.5 might be similar to 17.5 4 cell for now but I'll bet money that this will change as we figure out the whole LiPo deal. Mixing the two at a club level is fine for now but it will need to be split for big races.
Mod is what it is. This is the only class that should be able to mix LiPo and NiMh. Set a weight for LiPo and go race. We've had split weights before in sedan for 5 cell and 6 cell and that should work in 12th also.
Mix single cell LiPo and 4 cell NiMh at a big race and you'll see this ---->
and this ---->