[[M2S]moe]

I'm guessing your players have accounts and must login, so, modify player accounts to add a new variable: pMedic.

Make an admin command to make this variable pMedic = 1 for a player you choose.

Then under the callback OnPlayerRequestSpawn:

if(classid == MEDICSKINID)
{
if(PlayerInfo[playerid][pMedic] != 1)
{
SendClientMessage(playerid,COLOR_RED,"You must be made medic by an admin to choose medic skin!");
return 0;
}
}