The Omega D-series enlargers are generally designed to print down to about 3-1/2x5" with any format, as long as you have the right lensboard/cone (the flat one for 50mm) and the appropriate lens for the format. When I can't get the head low enough, I just prop up the easel on a box (find one that's nice and square, so it doesn't cause alignment problems). To print smaller, you could use a longer cone than normal or there is a reduction bellows, but you'll still likely need to prop up the easel.
Using a longer lens is not a bad option though. The only potential downside is that the head will be higher and more susceptible to vibration.