Matt,

I have a built wheel with a GS hub (I know, yours is NT) and a 7-speed Suntour freewheel. It drops into a 126mm frame with adequete clearance.

Your axle is too short if either the chain hangs or rubs on the seatstay in the big/big combo or it doesnt extend at least halfway into the dropout.

Worst case scenario for your combo would be respacing of the rear hub. If your axle is too short you could gut the NR hub or buy a loose axle and transfer to NT parts over. If the axle is long enough but needs different spacers you could use extras from almost any brand of hub.

Jim