Hmm. Given your no bigger, no smaller mass rule, that severely limits what you can choose to do for either. I mean, usually what people would either want to change into is a bird so that they can fly, but most people are bigger than birds. And for the summoning, people tend to want to summon something they can ride on. Can't do that either. The thing you summon will either be too small to ride or it won't be able to lift you.
So yeah, the way I see it, you can either change into another person that has the same body mass as you, or you can summon a person that has the same body mass as you do.