Ah ok so Steindachneri can get quite big, some males upto 8 inches. If its definitely the Blue Acaras - I think I saw another thread with it being the Electric Blue which is a great choice.
If it were me doing this now, and I had a similar choice made with a Nicaraguan Cichlid vs my Mbuna tank, I'd go for a single specimen. Probably a male with the Electric Blue to get the best colours and then fill out the tank with other species, its one of the best things about a South American tank is the diversity and options you have, especially in a 300 litre. Maybe something like.
1 x Electric Blue Acara
9 x Annostomus Ternetzi
20 x Lemon Tetra
12 x Flagtail Catfish
5 x Red Lizard Catfish
Maybe some Pencil Fish for the upper third?
Just before I give up challenging you on the African tank
as an alternative you could do an Mbuna tank with loads of rocks
5 x White Labs (1m 4f) - White with black stripes in unpaired fins
5 x Saulosi (1m 4f) - Black and blue striped male, yellow females)
5 x Acei (1m 4f) - Blue bodies with yellow fins
5 x Rusty Cichlids (1m 4f) - Rusty red/brown with purple flanks
5 x Synodontis Petricola
These are all pretty common Mbuna and I think these 4 are quite a safe mix but the more you look its incredible. Compared to the American counter parts that are (controversial statement alert) sort of colourful, but also sort of brown... Mbuna are in the main part are rich, dense, solid colours. I have a male Metriclima Msobo Magunga that is black and bright blue and sometimes I don't understand how an animal is that coloured, its a metallic blue and matt black. But I digress
Wills