1) As long as you checked.
2) Horn Company having shields is a contradiction of their own lore as they would've more than likely utilized something so useful in such tight halls it's unlikely they'd have shields in their loadouts when even Havoc Squad doesn't (at least what little lore they have.) How would this group make DU more elite? The reasons you gave seems like its just what DU should already be doing. I get the RP stuff so no complants there. Anything to improve RP is a positive change imo. As for Special Operations, SO was added to 21st due to their direct work alongside Jedi Master Ki-Adi-Mundi during Point Rain on Geonosis. They also were not bound by a battalion since Special Operations exists as SOBDE and not a battalion. Not to mention SO was already on the server and was just being battalionized and at the time 21st had 1 subunit and SO had already worked alongside Mundi.
3) If they have no relation at all aside from colors i can't see they Devs working on models and paying for them just for it to be added. Especially a little late with the new update around the corner, you might not see this for awhile, and for all you know the next Doom could wind up not wanting it if DU winds up plunging again. If there was a suggestion everytime a subunit was full and someone still was interested the forums would be full.
I can't see this working with how DU is already supposed to be a liimited shield wielding battalion that excels in defensive formations and suppressing fire. They also got Havoc Squad which while made up, Still has some decent usage within the battalion. Horn Company is simply a basic Phase 1 trooper company we watch get almost obliterated. I assume this is why the 9 slots, But you'd need to compensate for that somewhere else as there is a restriction on battalions having a certain amount of unnamed trooper slots (ARC, ARF, SUP, MED etc) With some exceptions such as 212th 501st etc due to lore characters. I wish the best for you and your future battalion choices but i don't see this happening right now, especially coming out of the gates with the dire situation DU was just in. You may want to give it a little bit more time before you try to push for Dev work to be put into a battalion that just came back from the brink. For now, -1. I'll keep checking this post to see if anything people say can sway my opinion as im still in between.