I can't take the credit for beta G
It was Natalia that told me about it when I had problems with shrimps dying when they tried to moult, and it seemed to work for me too. She told me there are shrimp specific bugs that can kill shrimps and while there are medications you can use, things that boosts the immune system like beta G are better long term. My shrimps are in the same tank as fish and they are OK with it.
Bear in mind that shrimps are more sensitive to water conditions than fish. When you move them to the 20 gall, acclimate them like fish even though they've been in the QT at your house. Depending what fish you have in the 20 gall, you may or may not see the shrimps as they tend to hide a lot when there are what are perceived as potential shrimp-eaters in the same tank.
Shrimps like moss and moss balls. These grow lots of micro-organisms which are shrimp food. They graze them constantly. For commercial food, look at Hikari shrimp cuisine.
Shrimps shed their skins to grow so if you see a dead shrimp, it may be a moulted skin, check it to see. Dead shrimps usually go pinkish like cooked shrimps. If you do find a motionless pink shrimp, it's probably dead.
If ghost shrimp are anything like cherry shrimp, be prepared to suck them up during a water change. My shrimps show no fear of the siphon tube and I have to pinch the tubing to stop the flow and prod them out of the way with the end of the tube. I have to go through the bucket before I empty it. There are usually half a dozen shrimps in there and not just babies but full grown adults. With ghost shrimp being a bit bigger than cherries you might not have this problem but check the old water the first few times you do a water change.