Yes, I think most citrus are self-pollinators. Here, where nearly every house has a citrus tree or two you often see a tree all by itself, but covered with fruit.
It really depends upon the variety of citrus, when they will set fruit and ripen. There are types of orange that ripen as early as October, and some that aren't ripe until March. At our first house here, I planted 3 different orange trees, so that I would have a continuous supply for juice from fall until spring. Then of course we moved away just when the orange trees were big enough to start bearing . . and the new house doesn't have enough space. (well, can't complain, I do have a very nice big mango tree here)
For your whiteflies in the greenhouse, a soapy water spray is probably more effective than dormant oil spray. 1/2tsp. of dish soap to a quart of water, spray all surfaces of the leaves and stems, at least twice about a week apart or so. Then spray every couple of weeks to keep the beasties from starting up again. If you have a warm day, pulling them out or taking the hose to them for a thorough shower inside the greenhouse also takes care of a lot of insect problems, and the plants will love the wash.
