I was always under the impression that Nomad weapons weren't actually the real deal. More like adapted technology, weapons based on their design, or something like that. Meaning if you have Nomad guns, you actually have weapons based on Nomad tech. It makes sense since you really couldn't mount actual Nomad guns on a conventional ship.
As for police and the like using them, I'd probably consider that OORP. The LSF is another story, they could have certain units with the new tech for fighting in Nomad space, or against the Order, etc. So to me that would be acceptable.