I like to think of them as closer to Mollusca then plants. Do note while I love my SEF, they are one of the highest learning curve of any force in the game. Mind games are your friend.
Your list is fine for learning games. But, the Seed Soldiers are going to be a pain in the back side. They start the game in Seed-Embryo state don't auto-hatch until the beginning of your second turn. You can use a order to hatch early or a
ARO if giving the chance. But, until they hatch your back field is wide open. While you have 2 specialist the Seed Soldiers is basically useless for objectives do the Seed-Embryo state. And I like Seed Soldiers.
As for what next, any really.
Corax Hasht is nice for Seed-Links as he brings a Spitfire, but has the same downside as all Seed Soldiers.
Sheskin is fun, get a Nanoscreen and Multi-Rifle and can eat downed foes to get extra Wounds. She can also join any SEF link team, Gwailo are good here. Nanoscreens everywhere.
Caliban are good, they link and get Engineers for objectives. Plus, you can pay a point more to have them start already hatched. Only one model right now but you can proxy Seed Soldiers

.
Sphinx is my favorite
TAG, even with the new Camo/
TO Markers need to be the base size of model is not a big deal. As by the time you place the
TO Marker on the table it should be to late for the enemy.
Or any of the others.
Hope you enjoy them, and remember going full Combine Army only every takes 1 non-SEF Combined model.