We bred Rex and Mini Rex with 2 different does and 2 different bucks. In both cases, the mini was the female and Rex was the male. Yes, I know the warning about larger bucks, but the size difference between our Rex and miniRex was not significant. We did not have any problems at all with the births.
The result was all kits outgrew the mama by about 4 months of age. We sold them all, but I expect they ended up about mid way between a Rex and mini Rex in size. All of them had the correct hair for a Rex/mini Rex. As they were growing, you wouldn't know they were a cross until they exceeded their mother's size.
In our area, I think a lot of standard size Rex genetics have been mixed with mini Rex along the way, resulting in standard Rex that are not as large as they should be.