Minor nitpick, but technically it would be Genestealer Cults with Tempestus allies based on the way Brood Brothers works.
Regardless, I have some Scions sitting around from a scrapped Adepticon list from last year and have considered running the two together. My first thought was to include them into my normal semi-mechanized list and have the Scions drop in alongside Neophyte squads while the vehicles move up to deliver assault squads. The Taurox Prime has a very similar defensive profile to Goliath Trucks which helps with threat overload and they provide a dedicated anti-infantry/hoard platform role that neither Goliath really covers entirely.
I like your idea too of having them start aboard a Valkyrie and drop them in alongside ambushers. Might be a good way to enable a 7th edition-style ambush list since they can be up with the rest of the army turn 1.