This is my first post, and i'll show how to make bunker available(can load/unload) other unit(such as protoss/zerg unit, or even big unit)
First, go to Data, Abilities, Bunker-Load/Unload(Bunker) (U can duplicate it or edit directly). To the right, find (None) - Target Filters, double click, change Biological' state from require too allowed. Now u can load/unload any kind of unit u want.
If u want other race's unit available too, find (None) - Load Validators, D-click and remove value
If u think your bunker space is not big enough, edit the 3 last field as image: