Yeah, the Gang Specialists were one of my favorite features in SRTT, I was always disappointed that we could unlock the gangs in Gang Customization, but we didn't unlock the Specialists as Homies. But yeah, I would hope the Idol's Specialist would be very similar to Grace.
For the Panteros, I would expect someone like that guy from the CG Trailer, the one who has the confrontation with Kevin, then get's blown up by the Boss at the end. He seems like he'd be their Specialist.
As for Marshall, I'd imagine something along the lines of the Murderbot from SRIV. would be their Specialist.