I bought a nashbar mountain bike frame that has mounts for disc and V-brakes. I wanted to use V-brake in the rear, but the cable guide that goes along the top tube to the rear brake is too big for the cable housing. The cable stops just slide right through. Is there any way to get around this. Are there larger cables that will fit into a hydraulic disc brake cable guide or do I have to get the disc brakes?

You could just run the housing all the way to the brake, and there is some small clips to hold the housing in the cable stop

Seconding just running the housing the whole way from the brake lever to the brake itself. If you can't find an easy way to connect the housing to the frame, zip ties work just fine.

I have to use zip-ties on my wife's bike for the IGH shift cable and housing. It came with frame "snaps" for the downtube, but I replaced most with thick black zip ties. Works great.

A couple companies need special ferrules for the cable routing. Cannondale comes to mind first, they tend to need these little metal or plastic adapter ferrules and then a regular one inside of it.

Run the housing from the lever to the noodle through the hose guides. Hydraulic brakes use hoses, not cables.

Right, but the OP wants a v-brake on the rear. I have to imagine there are little ferrule inserts you can put in the hose guides to step it down to cable housing size. Check with your LBS.

+1 on running the housing all the way back to the brake caliper. Older bike were often cabled this way. I have an '83 Trek that has top tube cable guides, not stops, for the rear brake so the housing runs full length.

If this is the same frame I bought there should be two plastic tube-like (ferrules?) adapters for that included with the frame. The guides are designed larger for hydraulic brake's hoses and you're supposed to use the adapters for regular pull cables.

If not, you should be able to get those from any bike parts vendor or LBS.

Adam

Look at the adapters that LooseScrews.com has. They have a multitude of cable adapters

+1 on loose screws or biketoolsetc. Both excellent source for random-ass parts.