Both the 201 and the 15 (and the 66) are standard low shank machines. The specialty foot for the 15 should work fine on any low shank machine.
The 15-125 is the 15-91 in a different coat. Not that there aren't differences between machines of the same model, there definitely are. It would be interesting to know if those differences are primarily due to one having been a well used and maintained vsm.
For advancing that quilt sandwich under the foot, try a Davis Vertical Feed treadle. It has a high/low lever for thickness of material under the foot.