I don't have anything that would really want to ride in them though.
You don't need to put anything in the serpents - they are a self-sufficient machine gun platform, best in the game.
125 Waveserpent with Scater Laser and StarCannon
This is an impossible combo, since the Serpent turret can only be a twin-linked weapon of one type, and the bottom catapult exchanges only into a shuriken cannon.
Regardless, you really should stick to the most effective combo, checked by generations of players - Scatter Laser\ Holo-fields. Very good save from jink or from being behind ruins, and maximum twin-linked anti-infantry firepower.
Screening your boats is also unneccessary - their firepower is neglectible anyway, and they can safely jink into a much better save. Unless it's some random lonely shot that you want to take on a 4+.