I have 2 fixed gear mtbs. One is an late 1980's Wheeler with horizontal dropouts, the other is a s/s specific frame with track fork ends. If you are determined to make a fixed mtb there are several options.

Get a s/s specific frame like the on-one or the outcast26 from bikeisland.com
Use a frame with sliding dropouts
Find a frame with horizontal dropouts
Eccentric hub
Eccentric bottom bracket adaptor.

Both my bikes are fast and comfy. One is set up as a winter/rain around town bike. 42x18 with full fenders, moustache bar and 1.25 slicks. The other has 1.35 slicks and is set at 46x16. No need to worry about tire selection, 26" slicks go as narrow as 1" and can run upto 110psi. The biggest issue I've had is the rear wheel. Not a lot of options for fixed mtb wheels and hubs. You will need at least a 130mm hub, 135 is better. Not spaced to 135, but an actual 135. Road hubs do not work on mtbs. Or use the wheel you have and get a surly fixxer.

700 wheels will work with brake adaptors, or a disc wheel works as well.