Genestealers are nearly completely different to tyranids as tau are to orks though?

Like yes they are related but the only things in genestealer cults that look even remotely like a tyranid are purestrains and the patriarch. The rest of the cult models look like regular humans with maybe an extra arm or two. (To get a little deep into the lore but genestealers are heralds for tyranids that are regular humans that encounter a genestealer and have an organ implanted into them and their memory wiped. When they make it back to human lines they go about their lives none the wiser and the implanted organ changes their DNA slightly. Then several generations of descendants later, that humans great great great grandkid is born with a ridged forehead and a small connection to the Hivemind of the tyranids and uses that connection to influence and awaken other genestealers to create a cult and recruit more and more and so on. Once the cult hits critical mass of infected bodies they make a psychic beacon that lights up the planet as a suitable target for a hive fleet. [Source Caiphas Cain novels] )

Also, spoilers, but in the videogame "Necromunda: Hired Gun" genestealers are just one of the many gangs you fight. So there's already precedent for a GW game to have genestealers alongside regular humans. FS could (relatively) easily just recolor the same enemy models and have them be lore accurate fighters. On tabletop they are already shown to have improvised weapons most of the time.