I haven't played with the Imperium a whole lot, so my opinions are rather limited. But here goes.

Elderly Swordmaster
Let him affect any creature you control, not just Imperium.

Soldier
Needs a little something so as not to be strictly worse than Valedune Arborist. I'm still thinking on this one.

Mos'k Ranger
This should have +1 Speed. This is very close to being playable.

Brigade of the Righteous
This should have at least +1 Health.
*Side note*
I don't really like the feel of this card. I understand what it's trying to represent thematically, but I think it would've been more effective if it was a weak creature and read something like: "When you play Brigade of the Righteous you may morph any number of creatures you control into Brigade of the Righteous."

Slumbering Titan
Should only have 3 Sleep tokens.

Mos'k Psychic
This is the only Imperium psion that doesn't interact with other psions in some way. I'm still thinking on this one.

Corathian Javeliners
Should start with an Ammo token. It's no Craig Scout.

Gunnr Valkyrie
Should give enemies only 3 flux.

Shield Generator
Now that it's been nerfed, I think it should be able to gain Armor again.

Garrison Ballista/ Trebuchet Emplacement
4 Range is insane. I think they'd still be good with a range of 3.

Acmar's Stabilizer
This should probably cost an additional flux.

Mana Dome
It got nerfed big time. It covers only half the hexes it used to. I think it should cost 1 less flux now.

Brimstone Powder
I think this could cost 1 less flux. At 3 domain, 2 damage is often just not enough.

Experience
Change the cost to 1IX.

Zap
This should only cost 1 flux. This is what Caprakin Warlock pays.

First off I have to say I'm a big fan of your "If I Ran the [Far Wilds] Zoo" series.

Comments:

Your suggestion for swordmaster might not make sense thematically: imagine what a pain it would be to teach a crocodile how to fight with a sword. Swordmaster can't currently target dimmed creatures (bug?), changing that would help a bit, but he still isn't very strong for a 2 flux 2 domain swordmaster, even elderly. I agree he needs some buff.

Soldier could use some special power, I agree with that. Arborist's ability is actually quite good in many situations, and especially in treent decks.

Mos'k Ranger could be buffed a bit too, but I'm not 100% sure about more speed though, as it's fast as hell already with a stride of your choice and speed 5. Maybe it could simply always have 2 attack? On the other hand, that would make it strictly stronger than gribralnock, which isn't a bad creature at all. Tough one...

Yeah, Brigade could use +1 hp.

Slumbering Titan: it isn't much tougher than a Blind Assassin or Bilgrin's, so one less sleep token might be justified.

Mos'k Psychic is actually quite good in my opinion for only 1 flux, even though it's 3 domains.

Corathian Javelineers definitely needs a buff, more than anything else. 1 range isn't very useful on a weenie like that, because it will take a long time to take out a Hadarck's Fort with just 1 attack, and the guy needs to work even to get the range in the first place?

Gunnr: Agreed.

Shield Gen: Yeah something could be done.. It went from OP to too weak so fast.

I think Ballista and Trebuchet are ok.

Brimstone Powder is actually very good, I think. Sure, it's weaker than Volcano, but then again volcano is kind of overpowered in my opinion.

Experience is unplayable because of the double CI. But that's just because celestial imperium is too weak for playing only them, or only them for first 2 domain levels.

Zap to 1 flux, agreed.

Very good work, I agree with almost all suggestions.
Headshot gave a nice explanation on what I am thinking.

I actually think that Mos'k Psychic is the best 3 domain, 1 flux creature there is. While it could probably use a slight buff, the main problem is that it doesn't encourage the use of other psions, something every other psion that the Imperium has does to some extent. It just doesn't fit thematically.