My water is just on the acidic side 6.8-6.9. Love the tank it’s very similar to what I want to do. The ellioti look stunning. I’m not sure on the silver dollars as they look like the Colombian tetra just a lot bigger lol. If needed I do have the option of going up to a 600l. Iv decided on the oases high line range. So have those to options to use. Then the chocolate would fit with abit more room it’s 10cm wider. It is the fish turning around on there that worries me when it’s full grown. Would hate to cramp them up
I'm always going to tell you to go for the 600 but I'm a terrible influence and probably not an impartial representation haha!
A general guide with fish as a minimum is to have a tank that is 6x its adult length long and 2x its adult length wide. So actually for something like a Chocolate Cichlid you would want a 6 foot x 2 foot tank as a minimum. BUT there is a balance to this rule for me, eg for a really common fish like an Oscar a 4 foot x 2 foot or 6 foot x 18 inch tank would be ok because a huge number of them. I would even say for a Chocolate Cichlid an 18-20 inch wide tank is 'ok' but if you ended up with a particularly large specimen that hits 14 inches you would have problems.
Interesting that you are on the soft side of thing as that means ignore what I said above haha!
Unfortunately I would avoid the Ellioti as they are from a harder water region and you will have more success with South Americans. From your original list you have a nice community coming together but 2 fish to flag, Keyholes are really timid so be careful when mixing them with others as they may get really stressed. They should be a pale grey with dark keyhole marking but when stressed they will go brown and hide a lot. Festivums are a nice fish and generally have a good reputation, but I once had a really bad experience with some and they were really aggressive, they are a nice top third water fish though but if you got a Chocolate too they could encroach on each other a bit.
I think from the fish you have mentioned I would go for a combo of the Chocolate, Blue Acara (regular or electric blue) and a group of Geophagus Tapajo (stunning fish!) The Geos are a gregarious fish, so not strictly schooling but do really well in a small group of 4+, you could try 2 of the Chocs and Acaras but I would be tempted to keep single specimens as a pair of Chocs could take over even a 600 litre tank.
Columbian Tetras would look great with them, you will need to be careful as Chocs are super predatory when I had mine he ate quite a lot of tank mates I tried... some of them unbelievably so you will probably need the Tetras in near the start of the tank and add a small Chocolate last. If you can find them Hemiodus would be a nice addition to the tank, they are a long thin characin but get some cool markings and are South American, an other option would be headstanders, Annostomus Ternetzi are an awesome fish I used to have a big group in my SA tank and loved them. Marbled Headstanders would be an other option and the Strawberry Headstanders seem to be more common these days and again, awesome fish (but I've never kept them).
Catfish wise you would have a load of options, Flagtail Catfish are nice, Hoplo species are cool too. Pictus could work but might be too active and stress the cichlids as you have quite calm species in the mix here. Plecos and Whiptails would be great too, I go for Whiptails over Plecos because they produce much less poop but something like a Panaque would look cool in this tank.
Wills
400 litres is a great size for a cichlid tank so you will have loads of options. 
