We get citrus trees here with up to 5 different kinds, orange, grapefruit, lemon, lime and tangerine for example, on one tree. From what I've heard, they are just a gimmick, like blue orchids . . .
I have had at least 3 friends who tried a so-called 'fruit salad' tree. Everyone says that after a few years (which it takes for the tree to start bearing at all) they have ended up with only one type of fruit on the tree, and it seems that whichever variety is the strongest grower, it overwhelms all the rest and 'takes over'. Honestly don't know if this would happen with other fruits, but it sure is a possibility. I think it's a much better bet if you have room, to just buy trees of one good variety for your area. If you want 3 types of peach, buy 3 trees.
Your friends and neighbors are a good resource for finding what kinds grow well - and which taste good! - where you live. Your County Extension service, or local Ag university would also be great people to ask.
As far as dwarf trees go, I'm sure it varies but I have seen some beautiful dwarf citrus here that bear very well, plenty of fruit for a couple or small family from one small tree. A full-sized tree often gives you so much fruit you don't know what to do with it all. And yes, dwarf trees bear normal sized fruit. It's just the tree itself that stays smaller than normal because it is grafted on a dwarf rootstock.
Up in British Columbia at the house where I grew up, my Dad had a favorite dwarf apple tree that only grew about 5ft. tall but it would bear a nice large box of apples every fall after it was mature. Cox's Orange Pippin, as I recall . . . an old English variety.
