I don't think the ships roles of many of the mercenaries are good for them. Almost half of them I wouldn't hire because I don't think the combat AI would use them well. Many of the early ones stay back when the should be at the front and later ships are in front when they should hang back. So here's the ships I think have the wrong roles and what I think they should have. Some may be missing since not every merc is available in every game.

The Raptor
Tiny support
High Mass attack, armor, and 75% evasion
Should be an interceptor. Start up front and likely to survive.

The Nail
Small support
Missile, all defense, double hitpoints
Too well defended to stay back. Should be an escort.

The Bludgeon
Small support
High beam, all defense, ignore 30% of damage.
Too much damage to hold back. Should be assault.

The Sabre
Small support
Beam, all defense, extra beam range, ignore 75% beam damage.
Should at least be assault to be in the fight.

The Bitte
Medium escort
All attacks, all defense, 50% bonus moves to fleet.
Well armed and protected but it has a support ability so you don't want it being the first choice target of nearly everyone. At most should be capital.

The Retribution
Medium escort
High beam, all defense, +50% beam range, +50% accuracy, +50% evasion
Yes, the evasion and defense is good but it has so much attack potential I wouldn't want to risk it as an escort. May be okay as is, but capital seems less likely to end in heartbreak.

The Praxis
Medium escort
Missile, high beam, all defense, mal ideology for each win
Its defense isn't that good and you want it to survive a long time to acquire ideology points. Should be capital.

The Heptium
Medium escort
All weapons, all defense, +30% fleet weapon range, +30% fleet fire rate.
Fleet wide bonuses should never be escorts. I'd put it in the back as support.

The Wraith
Medium support
High kinetic, all defense, fleet wide repair.
It has fleet wide bonuses, but it's well defended with high attack, plus the repairs. It can survive as a capital and make use of its attack.

The Compact
Large escort
High beam, all defense, +150% credits to planet.
Not that well defended and has an out of combat effect. At most should be capital.

The Linked Aerie
Large escort
All weapons, all defense, 13 fighters.
Who wants a carrier taking fire for its fighters? No one.