The Mistresses are the elite of the Dungeon faction. It's better to remove the Hydras altogether and replace them with Manticores, but only decent Manticores, not the neutral ones.
No, the Hydras should definitely not be touched. It's not uncommon for Stalkers to be banned in tournaments or agreed-upon games. And what do you do then without upgraded Hydras? Of course, you can try to compensate with Raiders and single Fury units, but not always.
Added 3 minutes later
As for the Mistresses, I agree with you - if you upgrade them to level 5, it will be too overpowered.
