Please do some reading as it appears you are well intentioned, but a bit misinformed.
I would highly encourage you to not add anything more to your current set-up and consider returning a couple of inhabitants. In particular the mandarin and angel which long term need a much larger aquarium.
The angle will just get to big and the mandarin will die of starvation due to lack of food. They do not readily take prepared foods and need pods
to survive. I am not the only one that will tell you this.
Please take the time to read this thread which deals with a very similar situation to yours:
It is very easy to get excited in the hobby and want to add new species, but this is not the best thing for the fish. The key to SW
is patience and go slow. I would encourage you to think about saving the $ you are spending on live stock and save up for a larger set-up which it sounds like will better meet your needs.
In the meantime do as much reading as you can. The challenge with this hobby is so many rush in and burn out quick when they meet adversity.
In the meantime 3-4 small nano fish with corals and LR
can make a nice setup. You really need to look at your 20g as a nano set-up.
Here are some examples of appropriate fish:
Again, not trying to beat you up, just trying to save you from mistakes I have made do to lack of knowledge in the past.