According to this article the policy is permission needs to be given by the player who's jersey is retired. Sterling Sharpe would not give Alshon permission to use his number and has denied the request on multiple occasions.
Not sure if the policy has changed but I don't believe so as there is...
What do Specer Rattler and Jordan Strachan do? I assume they still are going to wear #7 since I haven't seen otherwise. I guess they are kind of grandfathered in since it was already selected prior to the retirement decision.