For Veteran promotion the only thing that matters are stats. You will need:
60 TUs
80 Stamina
35 Health
60 Bravery
50 Reactions
80 Firing
55 Melee
If you use IGMA, there is a stat string mod that is telling you which kind of training they still need (except for TU, health, stamina and firing, but they usually come with time)
Officers promoting to Veteran Officers require the same. Veterans can always promote to Vet Officers. But you can turn Scions into Scion Officers too which are mechanically the same. They can gain rank 6 in the latest version too.
Also Scion's stats aren't so bad in general.
While Krieger and Catachans are interesting, I am more interested in mechanically different units than a few stats changes. Elysians can fly, Priests can buff, the Vindicare can pierce energy shields with the correct ammo.
If someone is willing to draw the sprites and give a few good ideas as to how integrate them (how to get them, what makes them special, what's required to unlock, ...), I am willing to put them into IGMA too.