I would not keep a stallion that was not.
Even Rabbit, who never runs with the mares, is a perfect gentleman with the foals, in fact he will allow them to do anything at all, bite him, pull his ears etc.
I have just bitten the bullet and turned Carlos out with the set mares and foals- he is a paragon with them- they push him out of his feed!!
I saw him yesterday flatten his ears and swish his tail at a colt and the colt just looked at him and carried on munching through the bucket!!
Poor Carlos, he is a pushover!!