You're not going to like this. Sorry but I have to say it.
I sincerely think you've rushed into this. A SW tank should be in the 1.024-1.026 range of specific gravity. My understanding is that you've been increasing the numbers to attempt to turn a brackish 20g into a SW nano but that you had not yet reached it nor tested to be certain it had enough SW bacteria to handle the dentrification cycle. Do you have a SW test kit? If so what are your test numbers?
The coris wrasse in my opinion needs the 30g minimum, it is going to need to be protected from jumping, and it is going to need at least 2x day especially at the diminuitive size your's is at. When it gets to be an adult it may go after your crustaceans, it might not but it is a risk you'd want to consider. It is a one of the best coris for a reef tank yes, but like any other fish it shouldn't be bought before knowing about it's care.
So right now you have 3 tiny fish yes, but those fish are all going to grow. The cori is going to be close to 5", the damsel will stay under 3" but don't let the size fool you as it is a damsel even if one of the less aggressive ones. The mono gets up to 10" and my understanding is that they prefer to be in groups and don't move well as adults so it is recommended to get a very large tank and have a shoal so a loan mono isn't stressed and hiding.
It is great that you took the amano shrimps out of the equation but instead of buying new fish to add to your system how about considering the many posts from several posters in the marine forums replying to your questions and saying that the mono needs a better home? That a SW tank needs properly cycled and water parameters made suitable for livestock? We all would love for you to succeed and are obviously willing to answer questions but sometimes you might not like the answers.