Honestly, I don't marry any of my children until I raise them and see who's best at what. I'd say to keep units with the same high stats together to compliment each other, so that the unit as a whole has godlike magic or skill. I also recommend pairing together those who have skills that compliment each other, like one unit having a ton of offensive skills (importantly Galeforce, alongside things like Lethality and Lifetaker or other activated skills) and pair it with a unit that never dies, like a unit with all of the breakers, or with Aegis/Pavise/Sol, or with Vantage/Vengeance. It is highly important (I think, at least) to keep Armsthrift on as many people as possible, and maybe even to keep the Armsthrift users together. If you have a really easy map you just need to clear out, one pair of armsthrift users should do the trick without losing any money die to weapon usage. I recommend having at least one pair where they both have armsthrift and galeforce to complete levels like that.