All my fish are cichlids right now. I'm into the breeding and parental care. Central Americans, as they suit my harder tap water.
I've never been one for Severum's, not sure why. Beauty in the eye of the beholder I guess!
If I was you, I'd be tempted by 4 or 5 small Senegal Bichirs only in the 340 litre with a commitment to myself to upgrade the tank size as the Bichirs grow. What's the foot print of the tank, 4 foot in length, and the rest? They might look tiny in a 340 litre at first, but you'd be setting yourself up for success.
My Senegal bichirs were awesome, always regret moving away from them. They were so active, not just bottom dwellers as some people think. The beauty of having just them is that you can tailer their diet 100% to their needs, no compromising, that's the beauty of species only tanks.
But I suspect you are attached the Severum idea, fair enough.
Have you thought about dither fish instead of other cichlids? Certain types of tetra's for example, or things like Corydoras catfish or similar? Do you want a cichlid tank or a traditional community tank of smaller fish with one Severum on patrol? Obviously, not such small fish that the Severum dines out on them. I don't know if Severum's are happy being a lone wolf as the only one in the tank, maybe, not sure.
I'm not sure what I'd do cichlid wise with a Severum, always such a gamble with Cichlids and how they will interact with each other, unless the tank is huge. What about Rams? Or Rainbow cichlids, although they are harder water ideally? Blue Acara? I think my concern would be breeding with any multiple cichlids of the same kind, unless you have a back up plan for removing a pair and breeding them separately if they pair up in the main tank?
I'd still have a think about your water parameters. I wouldn't keep Severum's unless I had soft water, but loads of people don't seem to give a to$$ about water hardness.