As far as I am aware, new teams get new players. The retired players aren't used again unless they become free agents.
As for the second question, if a manager fires a player then that player will not become a free agent, even if he is good. The only exception to this rule, is that players that are on a national team that get fired WILL become free agents.
Basically, national team players that are fired become free agents, all other players that are fired will become retired and not become free agents.