I think an Oscar with the other fish wouldnt be a great idea - a member on here had Pictus with an Oscar and one of them ended up in the Oscars mouth.... it was quite an ordeal but both fish survived - Star 4 was the member in that case you might find her thread but shes not so frequent here anymore. A 75 is the bare minimum for an Oscar as well so adding more fish with them in here might not be a great idea.
I like the way the tanks going so far
Blue Acaras are really nice, Im a big fan of all Acaras right now and trying to source at least 2 species for my tank, Krobia sp.Red Cheek and Laetacara Thayeri but they are by the by...
For a bit of red in the tank I think the best option would be a Rotkiel Severum - though the obvious choice is a Red Spot Severum.... or a Gold Severum but the reason I would go for the Rotkiel is they are more natural looking where as the Red Spots are more artificial looking - they are just line bred fish but do you see what I mean? I always think that a Red Spot would look great in a tank with an Electric Blue Jack Dempsey but again Im drifting... - I also think that a 75 again is a bare minimum for the Super Reds and Golds as they do get larger than the Rotkiels.
The other option is to mix South and Central Americans which personally I dont like to do but this mix is quite safe IMO - Firemouths
Plenty of red and not too aggressive and they make a great mix with the Blue Acaras - the other Central that you could think of is the Rainbow Cichlid.
As for tank mates I like to keep American tanks American, so that would be things like Spotted and Marbled Headstanders, Disk Tetras like Flame Tetras or Diamond Tetras, Lemon Tetras etc - maybe a stock list like this?
1 Blue Acara
1 Rotkiel Severum
2 Rainbow Cichlids (they do better in pairs than singles and are pretty calm and placid when they breed)
5 Spotted Headstanders or 5 Marbled Headstanders
10 Flame Tetras
4 Pictus Catfish
1 Sailfin Plec
With a good external filter like an FX5 and regular maintenance and I think this tank is very maintainable - if you wanted to lighten the stock somewhat maybe leave the Headstanders out?
Wills