I've had this guy in a7 gallon by himself for a couple months,
and I want to add him to my 20 gallon tank.
I have a tomato clown,
bicolor blenny, small 2 inch zebra blenny and a golfball sized sea urchin.
I skim heavily,
feed just enough so bioload isn't the issue plus I'm upgrading to a 29 sooner than later.

How would this guy do with my other fish ?
Would it harass them, or since my other fish have staked territory find his own part of the tank and not bother my other fish?

If it's a risk,
would keeping him a breeder net and observing how he reactos be a good idea ?

Or are they like damsels,
soemthing that shouldn't be trusted ?

It will likely fight at least with the blennies. Many Psueudochromis can kill/harass fish larger than them. 20 gallons is too small for other fish to be safe. They are best kept alone, like the 7 you had it in.

I added him a couple hours ago.
I'll go about catching him with a bottle,
for now he is shy enough not to harass so I'll withhold food then trap him.

Hahah!
You're right !
He chases the zebra blenny,
leaves the bicolor alone, and the tomato clown chases him.

Have a bottle trap and I'm rying to catch him.