The negative carrier on this enlarger should be a glass carrier. At least mine are. This negative carrier does have masking for smaller formats built into the carrier. I print 4X5 on mine all of the time.
As a side note, if you ever want to go to enlarging 8X10...Michael Mutmansky has converted one of these 138 enlargers to an 8X10 enlarger. Quite simple to do with the addition of an Aristo 8X10 cold light head, a negative carrier, and suitable lens. The negative stage of this enlarger has enough metal to allow opening it to 8X10 format.
The 105 lens will handle your medium format...the only thing that you will need if you want is a 50 mm lens for 35 mm. Good luck.