If your Arkie is burnished it will be much higher than a Coticle. IMO. But a coticule is a cutting stone (Cut and finish) where an Arkie is a polishing stone (Finish only). An old Coticule is a good thing to have. As far as the dished stone, I would check and see if it is a BBW and lap it flat, removing what is left of the yellow side and use it as a BBW. Not much yellow left to be of much use.

But as Steel said, Learn one stone and figure out what it does buy spending a lot of time on it. Trying different things. Then go to another stone and learn it. It's the best way to learn stones. Now if I can just do that.