I´ve been playing your mods for a while, definitely hoping for a broader update in future like the older one with many races - great stuff..
You can downsize the portraits in batch mode using something like Irfanview. Took a couple of minutes to do, here´s the result
https://drive.google.com/file/d/1QSxbImm_4oY407FZMAKTkM-74kiLp7_U/view?usp=sharingRegarding the Mirror Darkly canon music, there´s more than you might think - I used them in combination with the ENT music for the homeworld music mod. In this case I can make a bunch of submods per race - I´ll make a mirror/Ent music mod and can make more later if you like it. I´ve made selections of the following (All have combat and idle playlists):
ENT: Starfleet, Klingon Romulan
TOS: Federation, Klingon, Romulan
TMP: Federation, Klingon, Romulan
TNG/DS9/VOY: Federation, Klingon, Romulan, Cardassian, Borg, Dominion
The ENT series has some Mirror music too, so could separate them into two or go back to the originals and gather more. (there´s literally thousands of music pieces I had to gather while making that music mod seriously).
Great to know you´re still working on the TNG mod, the older one was awesome and although it needed some balancing work it was one of the best mods for this game imho. I think mods that follow such big universes as star trek have the most potential and I´m happy if I can help out in some way. Could be amazing to also integrate various other mods to flesh out a "Star Trek Total Conversion".
Regarding the viable enemy for the ENT or TOS - I would have to say Klingon without a doubt. Mainly this is because Romulans were barely in the show in comparison to the Klingons who have a much more distinctive and fleshed out culture in the universe. Even in TNG and further which brought the Romulans into the story much more, they still didn´t go much further than they´re the "Mirror Vulcans" who like their ships to be the colour of their blood.
I´m not into modding this game yet, but have been modding other mods and learning in the process. Maybe you can try taking a mssion mod and hacking it up. Maybe start integrating other mods if possible, connect with mod authors who are into this game and probably like star trek too. I think there´s a big opportunity for a great star trek mod here to build on your already great work.
Anyway, enough typing - I´ll get working on a collection of music made for Mirror Darkly and ENT.