If you put 40lbs of rock in the new 20g I don't think there will be much room for the 3 fish you mentioned AND a dwarf angel. In the best of circumstances an angel requires a 30g tank to begin with.
is a good start, I would say wait though on the mandarin, for the pods
to start to multiply and for them to build up a colony. A mandarin can eat up those 2000 pods
in less than a week.
I don't think you will have an issue with bioload as you have this plumbed directly to you 150's sump right?