If a guy last season was drafted in the rule 5 draft and thus was added to the Big league roster of the team that picked him, who then in turn traded him to another team who also kept him on the MLB roster.

This season they kept the player on the 40man roster but have him in AAA. If I trade for him I want to put him in AAA, do I have to keep him on the 40man roster too though? I was just going to assign him to AAA. 

The rule 5 issue is irrelevant. Rule 5 status has no impact beyond the season in which he was drafted. 

The question is whether the player you want to trade for has options remaining.