Thanks to all for advice! Our big boy, Kirby (a Paint built like a semi) lumbered right in, lots of snorting and looking for food. Sally, the mule, was more timid but they settled down after awhile. Kirby had no problem going into his stall for the evening hay, but Sally wouldn't. She'll figure out she has to if she wants to eat - Kirby is the dominant one so they'll work it out.
When we were building the stalls, we were thinking "this one's Sally's, this one's Kirby's..." and of course they chose the opposite.
So far so good. Appreciate you were all here.