Having played the game when there were darn near no special units, I would commonly purchase Type 2 Engines and call them Rehab Units because every engine added reduces the time on scene. Sometimes I'd call a Mobile Air Unit Rehab.
I've done similar with Engine-Ambulances. They're just ambulances in my realistic builds, but with firefighters, counting as an additional engine.
As for RIT, it's not necessary in the game. There isn't a firefighter down or rescue scenario or mission. I hope there never is. As said above you could call a Rescue-Engine, or whatever you want a RIT Unit and use the Custom Typing to call it that.