I think you would be ok doing as you said.  It's pretty common around here to see white/yellow corn right across the fence from each other

As far as how much distance to avoid cross pollination- kind of a loaded question because you can go in the middle of a quarter section of white corn and find yellow kernels on ears.  But distance where you would get cross pollination significant enough to throw your sample off- probably the 12-24 rows you stated are good enough.