Regarding the use of Glass Cannon I haven't tried it without the synergies of Control, summons and Health increasing skills, if it is still as strong as you say I can see removing it from the list of allowed skills.
Quick numbers test
On hard, at level 21
A Glass Cannon character with 13 Constitution (and no other stat allocation) will have
415 HP, 7 starting AP, 12 recovery AP, 20 max AP
A NON Glass Cannon character with 13 Speed (and no other stat allocation) will have
435 HP, 11 starting AP, 10 recovery AP, 12 max AP
The Glass Cannon Character is considerably better off, AP-wise. Their first turn is weaker, but they gain AP faster and their cap is much higher. Their HP is about the same. Any +Constitution gear will be much less effective on them, however. If you aren't planning on stacking Constitution on them anyway, then this won't make much of a difference.
