I am planning on running the second session on TEP, and is using these creatures as an add-on;

STOP READING NOW IF YOU PLAN TO PLAY in The Emperor Protects:

Genestealer Hybrids. Can anyone suggest the appropriate stats? Thanks.
My idea is to use them in a cave-in in one of the promethium caves. When
(and if) the marines rush to help the (hybrid) workers stuck in the cave-in
they are in for a surprise. Only the front of the head and one (normal) hand
is visible among the rubble. It pleads "I'm schtuck...! Please help!" Then
another appendage comes out when the helping marine is close. And it is
not a hand... What do you think? Maybe a bad idea? I don't know.

How tough do you want the hybrids to be- do you want them to be an Elite level challenge like the Genestealers are? A Horde level, easy obstical that's there just for a scare, or do you want them to be something in between? How many of them will there be? Is there an Apothecary in the group?

If you want them close to the level of genestealers I'd give them similar stats but swap out some of the visuals so players see what they're fighting. Something horde level simply make a cultist horde with some extra traits to make them more hybrid-ey. Something in between go, well, in between- personally I'd keep the wounds/tougness/armor of their superior cousins but reduce the weapon skill and damage traits (like eliminating razor sharp- give them some sort of non-primitive melee weapon that's 'okay'). The multiple attacks tend to 'scare' PCs, but the low WS will prevent them from being overwhelming to the group. Also, the reduced damage means that they will likely only inflict scratches on your PCs rather than critical wounds. That said, if they have an apothecary who is good at his job, upping the total damage from the encounter hurts a lot less because chances are most of those wounds will be healed up before the next fight.

Thank you very much for the answer. The short and hurried reply
(I'm playing this today! So all answers are most useful NOW.):

1. Yes the party includes an apothecarion.

2. I want the hybrids to be hard but not as hard as the genestealers.

3. I'm probably blowing the cover for the Stealers if using this, but I was
going to let slip one single Stealer as the "leader" for these hybrids, and
when the marines defeat that one, they will (hopefully) think the infestation
vanquished, but I'm not holding my breath.

4. One very vital info: all the marines have taken the decision NOT to wear
power armour when walking around in the Haistand settlement, perhaps
they will don their armours when venturing outside. Right now, the next part
is going Diablodon-hunting in the canyons. Also w/o power armour.

Ok, so now I'm thinking:

Ws BS S T Ag Int Per WP
38 - 40 60 50 40 50 30

How many of them are there? How many wounds and do they wear any armor? Do they have any weapons? Are the weapons primitive?

10 Wounds without armor with toughness 6 will probably die in the first punch or two. However if these guys are ambushing the PCs and get off the first hit, and there is say one per PC, but if they're ambushing and get off the first attack that may be all they need to do to give the PC's a bit of a jump.

19-20 damage with no pen will do, on average, 1-2 wounds per successful hit on a marine in armor. Consider giving them a big weapon, or toss on unnatural strength so they can do something like 1d10+10 non-primitive damage (mono edged swords or the like). That will mean average hits won't do much of anything, but a higher end damage roll will scratch the PCs. Factor that over a couple of hits in a turn and you'll scratch the PCs and give them a shock (and a distrust of locals). I'd adjust the damage based on the wounds/armor/tougness you give them, as a foe that lasts 4 turns will hurt your marines more than one that just gets the ambush round.

If you want more damage ('cause they have an apothecary) consider giving them some sort of tyranid weapon that can dish out more damage, or give them a moderate penetration to take some of that pesky armor out of the way.

Remember that the trip to the infested village should clearly be a combat portion of the mission. Unless the PCs are really trying to prove their badassery they'll go in with armor. If they do NOT, you'll need to think careful about how you want to play it as a GM- genestealers and the like are deadly to a marine in PA, they'll be even more so against a marine out of PA. So you will want to figure if this is a learning experience (meaning the PCs get their asses handed to them for being out of PA) or a tense but survivable encounter. If you want the latter, you may want to reduce the number of stealers to keep the party alive. If you want the former, go balls out and let them burn fate points.

1. There's twelve hybrids (I could only muster as many figs - I run very fig-intense sessions).
I usually also build up terrain for the encounters, with maps over every area. I've needed to
make a lot of maps for this adventure, as there essentially only one included.

2. They can easily have primitive weapons (only one of them has a spear). But it's easy to
simulate by saying that they're likewise equipped. But they also do have their third arm which
is a claw-arm very much like a Genestealer arm.

3. Other stats for the hybrids:
15 wounds, reinforced chitin (6 AP), 2 attacks (maybe three w charge), primitive weapon,
multiple arms. The genestealers also have; ambidexterous, crushing blow, hard target,
leap up, swift attack/lightning attack, lightning reflexes, preternatural speed, step aside,
multiple arms, unnatural speed, tyranid, as well as other talents and traits. Maybe I should
transfer something from this to the hybrids, the tyranid trait would be automatic I think?

The razor sharp attack type is another aspect of the genestealer, maybe the hybrids should
have that on the claw arm?

Yep, I remember the Grensvayl story. But they're probably gonna visit the promethium mines
first, and there's where I plan to hit them with both Karthas and his men BUT first of all this
cave-in with the "distressed workers" and a single genestealer.

Maybe I'll go with giving them unnatural strength + razor sharp + mono edged weapons.

As of yesterday session ending they're all now in PA, after parading the head of the dead Diablodon.