in short yes it is too small but if you get them young they can live in the 20 for the time being until you will move them into a larger tank. if you have another larger tank like 50G + then you can keep them in the 20 for now
They get along well with most fish so you can just try anything as long as it isn't too agressive.
It is a good idea to get 4 because they like to be in small groups but keep in mind they can grow pretty big just maek sure you have a 50G+ for them when they inevitably outgrow the 20G
Hi! I really wanted clown loaches also for a 30 gallon but realized it would be way too small as they grow well over 6 inches or more. What I got instead were panda cories. They are as much fun to watch and also enjoy being with a group of their species. They grow only about 2 inches big so you could easily have 4 in your tank. They do a great job of cleaning food remants in your tank too!
A twenty is just a little small even for four small clown loaches. They need caves and places to sleep/hide in. Mine went from 1-1/2 inches to 6 inches in a year in a 30G long. After they reach 6 inches they tend to slow down a bit. The two of them are in a 75G now and they use all the room they can get. when comfortable they zoom around the tank and scuffle with their friends. I personally wouldn't put 4 in anything smaller than a 55G with plans to upgrade to a 125 or larger in a year. Clown Loaches are really wonderful fish but can get really hugeover time.