Questioning if I can add new tank mates with my bonded pair of clowns-13.5 evo

[Haileyclown]

I bought a 13.5 evo tank with a full set up from a seller than was over the hobby. I made a bunch of upgrades as well and had to handle a crazy outbreak of algae from him.
However, this tank came with a pair of 2.5 year old clownfish. I was wondering if I could add another small fish like a fire fish or even a shrimp/starfish or will this “cramp their style” since they’ve been alone for so long. My one clownfish is a menace and loves to stare down my hermit crabs forcing them to go in their shells when they are doing their walks around the perimeter of the tank.
so I’m wondering if anyone has issues with aggressive clownfish and have any warnings for me that if I could add another fish or if it’s safe to say that I should just let my clownfish have the tank to themselves?
Also one of my hermit crabs does handstands where he pushes his shell against the tank glass and shifts down. Is this unusual, why is he doing this? I’ll hear him during the middle of the night with his shell tapping away lol.

 

[polyppal]

Prob fine to add another small fish, as long as it’s not a clown they should more or less ignore it

 

[Sharkbait19]

The hermit is just being a hermit…nothing to worry about.
You should be fine to add 1-2 small fish. I’d go for things like a gramma or goby that stays out of the way and utilizes the most amount of territory without getting in the clowns’ way.
Pistol shrimp and watchman would be a nice addition.
My biggest worry in a small tank for clowns is them turning on each other.
Shrimp would be fine.
Outside of asterina, stars don’t do well in small tanks.

 

[Haileyclown]

The hermit is just being a hermit…nothing to worry about.
You should be fine to add 1-2 small fish. I’d go for things like a gramma or goby that stays out of the way and utilizes the most amount of territory without getting in the clowns’ way.
Pistol shrimp and watchman would be a nice addition.
My biggest worry in a small tank for clowns is them turning on each other.
Shrimp would be fine.
Outside of asterina, stars don’t do well in small tanks.

Thank you, that helps a lot!