The UFOpaedia states that the Celatid:
1. appears to detect human brain waves and will move towards a human target even if well hidden;
2. has the ability to clone itself at an alarming rate.
So I decided to give them both these abilities
Now they can feel you outside of line of sight within 13 tiles from themselves, and every third turn (± one turn) they are gonna split out a clone. These clones are actually slightly weaker than the adult Celatid, and cannot reproduce yet, since I decided that the time they are gonna need to mature must last more than what combat spans. And since splitting itself approximately into two takes time, during the turn in which it clones itself, the Celatid has only 50% TUs.
This mod still has some quirks that I can't (or don't know how to) iron out for now, but they are minor.
1. Since the only way to spawn an enemy during combat is via a weapon, the Celatids are actually carrying around grenades that do nothing more than spawn the clones. These grenades can be picked up from the Celatid corpse. I did not assign any sprites to them, so you won't see them, but if you move your mouse pointer next to the Celatid corpse (in the Inventory screen, when standing on the tile with the corpse), you can pick them up.
There is no way around this for now. Fixed.
2. If the Celatid was killed during the alien turn in the end of which it was gonna clone itself (i.e. alien turns 3, 6, 9, etc (± one turn)), then even though it's dead, its clone will still appear in the end of this turn. This is gonna be rather rare because it can only happen if it's killed during the alien turn, and only every third alien turn to boot, but it's still possible (if the Celatid is killed with reaction fire from your troops or with friendly fire from aliens). There is also no way around this for now.
Other than that it works fine.
I would like to express my gratitude to Yankes for his script language and his explanations on how to use it, and to Meridian and Solarius Scorch for explanations on rulesets.
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Attached the version 0.9. The first of the mentioned issues is fixed.