I'am thinking about this myself and i'am considering using them for Cult mechanicus and Skitari. I want a complete army and not an tau ally.
I think the regular Demiurg infantry could count as skitari rangers or vanguard. Lots of weapon options and really gives you opportunity to show that Demiurg got better technology.
Toughness 3 is fine because they're rather small models and maybe the get/keep feel no pain to make them sturdy dwarfs. To add some ion cannon rifles I would pick something from 'Puppetswar' or 'Anvil Industries' miniatures/bits.
I want to go for the 'miners' theme and let them use machines with drills, breachers and stuff. I think with the upcoming
40k 8th edition, the army needs some good counter assault units and I want to include a bunch of mining machines to fill that part. Maybe use a bunch of cool walkers/robots with drills and use them as kastelan robots or whatever.
Thinking about something like this:
https://puppetswar.eu/product.php?id_product=322
I,am waiting for 8th edition to drop and then make up my mind.