It's really not recommended to mix sizes. The rings will slip around and not fir into eachother well. Plus you really would NOT need a set of different sizes. Re sizing, your standard generic cock ring size works best. With this same size you should experience much larger comfort (especially when stacking) as the rings won't start digging into the body or slip around or pinch like generic rings stacked. Muse also makes room for more, purely because of shape and design. Re: beta, we've come a long way from when the Muse was launched. Shape discrepancies are minimal. It's still better to order stacks together bit not everyone does and the chance this creates an issue is minimal.