stradibarrius:
A 50mm enlarger lens isn't designed to cover that large a negative. So if you want to print the whole 6x7 negative, all but the centre bit is likely to be unsharp and probably lighter (in the print) than the centre of the image.
For some 50mm lenses, the corners might even be blank.
Also, the higher magnification of the 50mm lens will mean for 8x10 or smaller prints you will have uncomfortably close working distances (the enlarger head will be too close to the paper).
You can use the 50mm lens if your intention is to crop heavily, and just print something like a 24mm x 36mm part of the 6x7 negative, but you need to be sure that that part is centred in the negative carrier.
Matt
P.S. for the same reason, you cannot normally use the lenses for your 35mm camera on your 6x7 camera. The 35mm lenses don't cover the film area.