From prison, not only level 20 and 30, but also level 1, 5, and 10. And if you can use heroes from prison without restrictions...well, then just remove all restrictions altogether.
no, it's specifically about prison only. On XL+U templates, the rules allow starting with any hero except Galtran. It's precisely on XL maps (except Jebus Cross) that these two are the most imbalanced.