Sounds about right. In terms of competency, Rubricae are essentially servitors.
One thing I'll warn you; this isn't a simple spell - it took a cabal of thousand sons, led by Ahriman - the most powerful sorceror in 40k to broadly fall under the category of 'mortal' - to achieve this. Creating a single Rubric servant is probably not too difficult, but you're talking about flashing it across an entire starship crew- tens of thousands of people. Essentially, not far short of the original. Replicating it is a massive, massive deal. Firstly, 'generic daemon' would neither know the spell nor be able to cast it. A daemon who is aware of the secrets of the Rubric is not going to be something the party is going to be able to summon....
....actually, scratch that. A daemon who is aware of the secrets of the Rubric is not going to be something the party is going to be able to bind or compel. You might be able to summon it but you have literally not a prayer of controlling it once you do. We're talking Chi'Kami'Tzann Tsu'Noi levels of power here.
Finding the secrets of the Rubricae is a major endeavour in and of itself - you're talking about the most potent ritual ever cast by the most gifted mortal sorcerors in history, the knowledge of which is (a) probably likely to cause nosebleeds and/or instantaneous insanity and (b) is pretty much the definition of 'extra heresy'.
Casting it even more so.
Lastly...how can I put this....Tzeench did like that spell. Magnus didn't, because it killed off 90% of his legion (or 90% of what was left of it after Russ was through with them). Tzeench actually intervenes to save Ahriman - the quote is "Magnus, you would smash my pawns too easily". Ahriman, for that matter, didn't like the Rubric. The spell was supposed to save the legion from mutation. He was aware there were risks, but It wasn't supposed to 'save' the legion by killing most of them.
To create your 'rubric crew', you're talking about summoning a massive amount of warp energy (as in 'sacrifice of nine hundred psykers' type energy) in a ritual you won't really understand, in accordance with instructions relayed to you by a daemon who's truthfulness you can't compel, and essentially hoping it does what you've been told it will.
Not that a daemon of Tzeench would ever, ever find a way to screw a supplicant on a deal...
....The tutelary say you need to drop your gellar field at this point in the ritual. Go on. Don't you trust him?
"But not this bit" - How are the explorers intending to delineate the effects of the ritual? If they say "the crew of this ship", that does include them, after all...not that you need point that out to them.
"You never said 'loyal' " - Ahriman and co can dominate the Rubricae because (a) they're brothers who were bound by gene-seed and (b) Ridiculously epic psykers. Assuming you manage to create invulnerable armoured automatons, how do they intend to control them?
"Actually...we've decided to invest elsewhere" - there are, you said, numerous Tzeenchian cultists amongst the crew of the ship. Supposing the daemon decided to favour them? Let the explorers exhaust or flat-out annihalate themselves (and the lower-ranked crew) and then his pet cultists from the officer's deck can walk in and take control of the ship.
"Yeah...I'm the other daemon." - A group of cultists will intervene in the ritual to disrupt it, in the hopes of embarrasing/binding/killing the daemon helping the explorers. Granted, the feedback from a failed ritual would blast the explorers (and their attackers, not that they know it) to spawndom, but they're only mortals - trapping a rival daemon in the derelict ship for nine hundred and ninety-nine years is worth it.
"Oh....you wanted it to be permanent!" - The rubricae fall apart after ninety-nine days. During a warp jump.
Edited by Magnus Grendel, 31 October 2013 - 04:03 AM.