In addition to some previous information, Sunstate new FO's are frozen no matter which aircraft they are assigned, 300 or 400, for 18 months from start date or 15 months from check to line. The only difference is the 400 guys are bonded for more $$. So if you are assigned a QLD base, those are the conditions. Not sure about Eastern EBA.
In regards to Command, and to clear some confusion, Qantas Link only operate under two AOC's, Sunstate and Eastern, not multiple AOC's for different capacity RPT ops. From the Sunstate side of things, the 400, 300 & 200 all operate on Sunstate's high capacity AOC regardless of their individual capacity. 500 Multi engine command time is therefore not required to hold a command. Initial command will most likely be on the 200/300 unless all the cards fall your way and everyone above you is frozen on type at the time of your winning command bid.
Haven't seen detail about Eastern's AOC but I would imagine it is also high capacity so as to operate 300's.