Re: Practice squad question
My understanding is that there is no right of first refusal for the team who currently has the player. Though practice squad players practice with their team, they're essentially free agents in regards to the ability to sign with another team.
There are some restrictions, however. Summarizing from The Daily Norseman, (1) a team can't sign a player from the practice squad of a team they're playing the following week, (2) if a team signs a player off of another team's practice squad, that player has to take up one of the 53 active roster spots for his new team for at least three weeks, bye week included, and (3) if a team promotes a practice squad player internally to their own 53-man roster, they must be counted on that roster for three weeks and be compensated the minimum salary for that time, regardless of whether or not they're released.
So yes, if the Rams put Onobun or Alexander on the practice squad, another NFL team could sign them away. But they'd have to put them on their 53-man roster for at least three weeks, it seems. Sometimes NFL teams will catch wind of a team who might be ready to sign one of their PS players, and they'll promote them to their own roster first. If I'm the Rams, I'd be more concerned about Onobun than Alexander. But the fact that the signing team would have to promote these guys to their 53-man roster in order to sign them away may be a good deterrent.