They all have a common type, the 700 and 900 are identical. The CRJ 200 and the challenger have a mostly common interface (to the 7 and 9) but some of the systems are quite different. The amount of differences training and checking is going to depend on your particular airline and the POI. Technically it is a common type rating.

My first airline required 6 months on the crj200 before they let you fly the 700 and then a differences ground school and PC in the sim.
My current airline has a 1 day differences ground school and 1 sim check which covers you for both the 700 and 900. IOE can be done in any of the variants (or all 3 even) and you are good to go.