Just because the Broncos are bringing in RB's for a look does not mean they will sign any. My understanding is that the Broncos regularly works out players on Tuesday anyway as a group. I.E. OL one week, DL another week, etc. It just so happens this week was RB's. So if that is true, let's not read too much into it.
With that said, Slayton is more of a Hillman type RB; smaller in nature, etc., so I'm not sure he would be a good replacement for McGahee anyway. I don't know if Joseph Addai is being looked at, but he would make sense if he is in shape just because he is familiar with the system.
What I think they will/may do is just activate Moreno and put him in the rotation. Maybe they promote Jeremiah Johnson from the PS, but I am not sure they have to.