The more support offered, the better the fit, since there will be less chance for the form to move in the bra. Bras with underwires tend to work better than those with just elastic because they offer better support for the form (the same reasoning that applies to natural breasts.)
Push-up bras, on the other hand, will generally not work with breast forms, since they need to push something FROM someplace that it is attached. Unless the form is attached to the chest, this type of bra will have no chance to create the desired effect of enhanced cleavage.
After having a mastectomy, the site of the surgery will be tender for some time. Radiation therapy and other complications may also cause other problems that make wearing a bra, especially an underwire bra, uncomfortable or just not an option. Depending on your situation, you may need to experiment with solutions that work best for you.
Leaning forward can reveal gaps between a form and the body with many bras. If you are planning to wear a garment with an open neckline, wearing a decorative pin to anchor the neckline to your bra strap can be effective at minimizing this effect.