26" vs 700c

Just a couple of counters - in your eg can you bar spin? and...

...with an MTB you dont have track dropouts to ride fixed... but then given the massive cost of all the fixed trickery I'd guess you'd save enough by using mtb parts to replace the dropouts.

Mountain Bikes (Specifically street and dirtjumpers whose frames I've been using as a parallel) have been throwing bars for far longer than people have been doing it on fixed wheels. The answer is yes.

On the second point, so many of the frames designed for street and DJ are singlespeed specific (Less shit to break right!). Meaning that with the correct hub they can be used either fixed of SS.