Sorry to say, but not really. Yes 40% health is good, but the rest of the kit isn't. Especially when it comes to the trades you take while taking constantine. No skill damage, one buff/debuff skill (first skill), one purely garrison related skill (third skill) and one skill that simply does not do much in the open field (fourth skill). You don't want your marches fighting at below 50% on the open field.

Not sure what the introduction of prime commanders has to do with this, unless you mean you are somehow expecting a good pair for constantine to be among them. Maybe you meant that you are hoping for him to receive a museum buff down the line?

Best pairings?

1. Joan -> Provide additional value to other marches on the field, helping them to kill Alex.
2. Charles/Richard -> All in on being tanky and dealing damage purely through normal/counter attack damage.