I used red guards decoupling mat under my shower and bathroom tile. It also say "recommended" minimum of 2" but I've had zero issues in 6 years with 1x1 mosaic tile in the shower. Personally I'd take the risk on the smaller tile which "might" be a problem later with cracking grout over not using a decoupler that "will" be a problem over that concrete joint.

The only other option really is to put an expansion joint in the tile. But that is going to be right in front of the toilet where you will feel it all the time with your feet. I'd throw down a membrane and worry about possible tiles moving down the road. Unless you are wearing high heels in that bathroom a lot with a point load on a single tile it's not ever going to be a problem. You'll lose any "warranty" by using small tile but honestly you are putting tile over a concrete slab... Water getting thru isn't really a problem.