This has come up a few times before as well.
First off, check the Rotary Wing forum at Aeroscale for a full listing of available figures. I recommend either the Academy OH-58D pilots or pilots and crew from Nemrod. There are no figures specifically sliding down a fastrope, you would have to convert some for that.
160th SOAR Blackhawks only use two fastropes, which creates a balance problem for the helo. It can be done, but would be difficult, especially at the connection points for the fastrope, which is on the cabin ceiling. They would need to be overly large and out of scale to support the weight.
Here are some pics of the same idea experimented by Avus.
He used a heavy wire and has it entering the underside of the helo, then he has an internal frame that supports it. The rod is still too big for a fastrope and doesn't attach at the fries rig which is on the ceiling of the cabin.
I recommend using a similar technique that I used on a customer's UH-1H Dustoff dio. I hid the support rod as a tree that the helo was flying near. The same could be done from a nearby building roof, antenna, or power pole.

More
here.Maybe posting over in the Rotary Wing forum will give you more results as well.
Good luck.