I spent a fair amount of time writing a reply, but got logged out or something and lost it all. OK, short version.
Class for psykers make no sense since all psionics (at least for your soldiers) is effected by use of specialized devices.
Yes, but the valuable things there are the specialised devices, not the soldiers. The gals can be anonymous, it is the devices that are important. The player will find (or train) plenty of gals that have the necessary stats to use those devices.
If devices can be limited to a certain rank or randomly generated stat, then the gal can become more valuable simply because the right stat needs to be used with the right device. For exampe, if the gal does not have the "Firestarter" stat, she cannot use fire-based devices. Once the number of useful psionic gals drops, the devices themselves can be made more powerful and valuable in turn.
I imagine a system where a hidden stat independent of Voodoo Strength/Skill (call it Voodoo Aptitude or Psi-Speciality) determines what devices the gal can use. For example, the scale is 0-X. 0 means no psionic devices ever, the gal just happens to have a strong or weak mind, but no psionic ability. 1 means "Firestarter". 2 means "Telekinesis". 3 means "Mind Control", 4 means "Demonblooded" and so on until we run out of ideas. There can even be a value for "Latent", they have powers, but you don't know what they are until they are unlocked.
The change to the game this would make is that, if they are made rare enough, if the player discovers an active psionic gal with poor Voodoo Strength/Skill, it would pay to train them up rather than to fire them and reroll some more recruits. If they are rare enough, the devices can be made more powerful from the start since the chances of fielding more than one or two has dropped.
The downside is that the player may be unlucky and never roll an active psionic. This could be dealt with. The first draft of this post had paragraphs dedicated to discovery, recruitment and rescue missions. I won't go into them here, just ask me if it sounds interesting.
And I ended with saying that I have a huge respect for what Dioxine and others have made. If it looks like I was being critical or trollish, I am sorry, I should have picked better words.