Once the draft list drops (or draft itself, I forget which) you will have to have 28 players in rookie league to get below $6M. Prior to the draft list dropping (or draft itself, I forget which) you can bid it all. If you don't have 28 players in rookie league when that event occurs and your bid takes you below $6M prospects the engine will invalidate your bid without telling you (it has happened to me).
EDIT: So if this is in Kinsella (only one of your worlds that appears to be pre-draft), everyone can bid it all.