The main reason I don't think a 20gallon long isn't large enough for an angel with more peaceful fish is the peaceful fish won't be able to get away when it throws a tantrum and decides everyone in site will get chased and bit. I HAD a lemonpeel in a 55 with 2 other fish a clown and a royal gamma. He was in there one week before I got him back out he was stressing out the other 2 fish. He was more active and agressive.
My royal gamma is very laid back and is out most of the time. He shows his mean streak by just opening his mouth wild if another fish gets to close to what he considers his territory.
You can do anything you want we are just saying from our personal experience and our own research what works for us.
Are you talking about a yellow clown goby or a watchman goby?