Hey there! ^^ So Rotom forms isn't a Boutique thing, but a
Department Store thing! From there, everytime you want your Rotom to change form, you buy the needed appliance. (Although returning to normal form doesn't cost anything, you still need to "buy"/register it.)
As for the Rotom dexes, that's a good question! Probably the mods will have a better idea on that
Hope this helps!