It would make sense that the USHL wouldn't allow players with signed professional contract in their league. It's speculated to be one of the reasons the CHL is considered a "pro" league. It would mess with NCAA eligibility which, of course the USHL would avoid at all costs.
If the player is 20+, they can be signed and play in AHL.
I'm not sure about if they are 18 or 19.
If you were drafted out of a non-CHL league you can play in the AHL at 18-19. I think the OP is asking because the Caps have Nathan Walker in camp from Youngstown. If he was signed by the Caps it would be going to the AHL/ECHL because no one owns his CHL rights.
As someone mentioned he can't be assigned back to Youngstown because then it would screw up everyone's NCAA eligibility.
Interesting that Walker came to the USHL after playing in a pro league (Czech).
True, and many former CHL players have played in the USHL. The keyword being "former". As long as they're not under a current contract and playing without pay, their pro status doesn't affect the amateur status of the others in the league. In theory, a former NHL'er could play in the USHL, but it's difficult to conceive of a scenario where a player goes from having an NHL contract to having no pro contract whatsoever.
This is similar to the reasoning for the mixing of pros and amateurs in unpaid tournaments (like the WJC) as well, except in those circumstances, the players CAN be under contract in a different league. Team USA would have a difficult time competing in the WJC with only pros or with only amateurs, so I think it's fair to make exceptions when playing for your country. The main thing is that nobody is getting paid to play for their country. That would be a deal-breaker for the NCAA.