Amos was a good player for GB and good in the locker room. His teammates liked him. He declined noticeably last year though -- he was never a speedster but it was starting to look like he couldn't run anymore.
At a minimum he can probably get everyone lined up, but I don't know if he's worth the contract he wants (he can probably get more elsewhere).

As for the rostered safeties, it is a wide-open competition and should be one of the better ones to watch. (OL, S, and WR) Someone will likely emerge and surprise, it could be this ISU kid and the guy from SF starting by week 5.