You should be fine with a 50mm lens.
28mm is probably too short anyway. I use 30mm for 16mm negatives and 25mm for Minox.
There are some 40mm enlarging lenses out there (they also work for 4x4cm), but unless you are maxed out on you column height, you probably won't need it.
I did some recent tests on how much masking is needed (like printing a 35mm negative with an 8x10 mixing box etc.) and it really can have an effect. If you have an adjacent entire blank frames, certainly it can cause flare (your highlights will go gray). If your adjacent images are dark then you may be OK with no masking and just centering your frame in a 35mm negative carrier.
There are a lot of ways to mask, event to the extent of making a new negative carrier of the correct size. Kind of depends on the enlarger you are using.