Appropriate size before adding growouts

[Willfisher12345]

Currently I have a full grown Jack Dempsey that I would like to keep with other fish. Most pet stores near me don’t sell adult cichlids so I will have to grow one out. I would like to know at what size do I need to grow the other fish out to live with him? The tank has a large cave structure in the middle that the Jack Dempsey can no longer fit inside of. Does the fish just need to be big enough to withstand a few nips from my jack before it realizes to hideout in the cave? Or does it need to be half or 3/4 the size of my jack to be able to fight back a little?

with this particular jack I have had a mix of keeping fish in the past with him.
The first fish I tried keeping with him was my 3 year old 5.5 inch rainbow shark when the jack was only 5 inches and he killed it the first night in the tank together(I thought I made caves that the jack couldn’t fit in but the shark could because of its torpedo shaped body but the jack got inside)

Other than that, over spring break(I’m a college student) I added 10 feeder goldfish and 10 feeder minnows(I bulked them up for 2 weeks in a spare tank) and he ate all 10 minnows but none of the goldfish and the goldfish are still living. I will be giving the goldfish to my friend who has a pond soon.
That all being said my jack has a strange temperament.
So I guess the main question is what’s the fastest safest size to add a fish into the tank with him?
also how effective is the eggcrate divider method? Will this actually help my jack accept the new fish or will it still be aggressive the second I take the barrier down? Also adding some pictures of him:
1 a size check from a few weeks ago
2 him sparkling in the sun today(sorry for glare

 

[Fishnerd360]

Fastest fish I could think of are silver dollars, but that depends how long your tank is. When I had a Jack Dempsey I learned he could still take down fast fish like giant danios and Siamese algae eaters.

 

[tlindsey]

What size aquarium is the JD in. Tbh the Jack Dempsey has claimed the aquarium as his territory so adding anything will be a problem.

 

[duanes]

Agree with the above, unless the tank is at least 100 gallons the chances are slim any other fish will be tolerated.
JDs are extremely territorial in small spaces, and living there as long as it has, it has claimed the entire tank as its sovereign territory.
I have watched JDs in nature defend a space equal to at least 250 gallons.
In the video below, at about the 1 minute mark, you can see a mature JD defend a space about the size of a car, in its natural habitat in Mexico.
The live bearers are ignored, but that's because they are very fast, and have ample room to escape, a JD normally only breaks off chase, at the 4 ft mark. Anything caught within 4 ft, is toast.
Cristalino

 

[Toiletcar]

What size aquarium is the JD in. Tbh the Jack Dempsey has claimed the aquarium as his territory so adding anything will be a problem.

X2 on tank size. Will make a huge difference.

 

[Willfisher12345]

Agree with the above, unless the tank is at least 100 gallons the chances are slim any other fish will be tolerated.
JDs are extremely territorial in small spaces, and living there as long as it has, it has claimed the entire tank as its sovereign territory.
I have watched JDs in nature defend a space equal to at least 250 gallons.
In the video below, at about the 1 minute mark, you can see a mature JD defend a space about the size of a car, in its natural habitat in Mexico.
The live bearers are ignored, but that's because they are very fast, and have ample room to escape, a JD normally only breaks off chase, at the 4 ft mark. Anything caught within 4 ft, is toast.
Cristalino

Thanks for the video it was super helpful in showing me behavior in the wild. As he is in a 55 gallon it seems like he’s gonna be the only Cichlid fish in there. Is there any chance I could add a smaller species of Pleco or a striped Raphael cat or would that be dangerous aswell?
Thanks for all the help

 

[duanes]

you might get away with a Pleco, JDs might not consider them territorial threat,
If it were me, I'd add it to the tank just before dark, Pleco's are semi nocturnal, JDs are not.