Get a young one (or five young ones) and you'll be fine.
Don't powerfeed and it'll live in a 300g for a very very long time. They don't grow huge unless the tank is huge and the food is omnipresent.
These fish are smarter than any others that I've owned. I've had several (both sexfaciatus and lussoso), and while there have definitely been behavioral issues, they have rarely been severe, and they have all been correctable. You can put one of these fish in a penalty box and it actually learns from the experience. You can re-scape and curb aggression easily. You can trick them into good behavior.
The one I have right now is in with African cichlids - mostly mbuna - and ever since I added a bunch of growouts right to the main tank, he has been the most peaceful fish in the tank. He'll chase a fish off if it comes too close, but it never lasts and never causes danger. These are the prototypical "overcrowding = peace" fish. You can put dithers in and they'll never pick on anyone to the point of it being a problem.
Before the fry went in, he had 12 yellow labs, 4 Acei, 11 demasoni, and a peacock in there with him. If the Acei and Peacock got into it (a common event) and one was battered, like many fish he'd start zeroing in and picking on the weak one, but that just meant I had to put the weak one in a hospital. Once he started getting really ornery with everyone, but a week in isolation solved that.
Knock wood, but it has been six months since he has shown even a hint of aggression.
With the current stock that you list, honestly, I would only worry about the severum. But again, all fish are different. Honestly, you could drop the severum, add a few dats, and then put in the distichodus and I wouldn't worry about anything. When I had even my meanest distichodus in with dats it never paid them any mind at all.
To be honest, in a tank of that size, I'd buy a small school of very young little ones. Much like many african cichlids, they'll keep the battles to themselves and be their own dithers. And again, as long as you don't make it your mission to grow them huge, they won't get that big. Once they hit a foot-ish their growth rate slows even more. I had a foot long one for over a year and it didn't grow at all. I think to get one past that size you need a huge tank, and to pretty much keep it by itself.
The other reason to start small is that their colors are easily the most brilliant before they hit 8". Once they start to get darker they're still cool fish, but you can get some real fire engine red out of them in the 4-8" range. This photo is not retouched and not on a high ISO. Just a generic flash from a point and shoot digital camera. The dark one is about 10". The other one in the foreground was more like 6".
The one I have now is roughly 6-7" and maybe it's the lighting, but he's not this red. His fins are, but his body is much oranger. I have raised him from a 2" anchor-worm ridden charity case to a healthy and happy (and peaceful, as noted, at least for now) specimen. And yes, I'm on the extreme end of the non-powerfeeding spectrum, but still, that should give you an idea about how they grow. You can get 9" in the first year if you really want (I've seen it done), but there's really no reason for that.
