If I was going for a properly uniformed army (like Space Marines...only as an example), I would stick to whichever miniatures were available that fitted the uniform...so they would probably be from the same manufacturer.
If I was going for a non-uniform army, I would select troops from wherever as long as they fitted the theme and worked well together... size/equipment wouldn't be an issue but if the styles were markedly different (and quality, for that matter), then that would play a part in the selection process.
I quite often use miniatures/vehicles from a range of manufacturers and if I think they fit together as a group, I'll use them... but I am only a collector and not a gamer.