2nd - the AT-TE is too outdated for combat at this point I'd think. It also doesn't serve any really special roles that would be worth using in the modern times of Star Wars that the AT-AT couldn't cover. I wouldn't mind the Pentastar Alignment having AT-STs and AT-STs being buffed with their canon grenade launchers and what not - a high damage low health vehicle.
While it was used more primarily in the Clone Wars, the AT-TE was still utilized by the Galactic Empire. This is because the AT-TE was so well built that many remained in active use even after the invention of the AT-AT (which was inspired by the AT-TE's design), typically in the Mid and Outer Rim. They provided additional firing support for AT-AT walkers if used in military campaigns but in most scenarios the walker found itself in patrol duty. If you are still skeptical, the latest known usage of the AT-TE was by the Restored Empire (13 ABY). An interesting thing that I've found out about it in the
Star Wars: Imperial Handbook is that they are sometimes outfitted with floodlights, underbrush grinders and flamethrowers. Everything considered, I think the AT-TE would make a nice addition to the Pentastar Alignment Roster, unless of course the mod team believes it would disturb the balance between the factions.
I do agree with you on the AT-STs needing their concussion grenade launcher, as it was what made them effective against armored vehicles. I would imagine that they weren't given it because it could have potentially made them too strong / cost-effective but that is merely a speculation.