Bicycle Mechanics - Roadbike brakes fit a Mountainbike tire?

Assuming the wheels fit in the frame, will I be able to use my roadbike brakes? They are Shimano Ultegras and the wheels are from an old Schwinn Frontier mountainbike/commuter type. I put some Nimbus tires on the mountainbike wheels and I want to try them out.

I think I already know the answer here but what say you? Need pics?

If you want to mix MTB and road wheels, you probably will not have success. First off is the hub spacing - road bikes use 126 or 130, while MTB uses 135, I believe.

Assuming the wheels fit in the frame, will I be able to use my roadbike brakes?

First hurdle is wheel diameter. Given the different diameters the (most common) MTB rim brake surface would end up more than one inch away from where the(most common) Road rim brake surface would be.
The odds that the road brakes would actually be able to reach the MTB rims are poor.
I've seen a guy using home-made brake arm extenders, but I'd worry about the actual pinching power one would be able to generate with that.

Next hurdle is hub width, but I'd expect that wouldn't be critical. First, frames are flexible and will readily handle a little mismatch. Secondly, (rear) hubs usually comes with a few spacers thet can be tinkered with to create custom widths if needed.

26 inch mountain bike rims won't reach up high enough for Ultegra calipers on a road frame to even touch the rims so that combo is a non starter if this is what you've got.

If your old Schwinn wheels are actually 700c wheels off a "mountain bike" like hybrid then they may work as long as the Nimbus tires are only around 1.25 wide. More than that and you probably won't fit them into the calipers since there's typically no room for fenders with the typical road racing caliper.