general rule for O's is roughly 55gal/200L for first then 25+ gals/100L for each extra fish (can ya tell i use american sites too much? :lol: ). as you mentioned never have odd numbers if you can help it, 99% of the time it just ends up causing hassles because one will get bullied as the others buddy up. try to aim for at least 5x turnover per hr when using a canister, im about to setup a 2200lph aquis on my 460L and i might keep my 304 running on it. oscars are very messy fish and need good filtration to match up with their output.
IMO four oscars would be fine in that size tank and you could perhaps add some dithers like silver dollars or something.
Severums make nice big, generally placid fish but IME my boy isnt as interactive as my O, close though. There are several large species of cichlids that are very impressive but from what IME most dont match the personality of the good old oscar. Jags are another possibility but to save heartache dont add tankmates unless your prepared to lose them unexpectedly one day- countless stories out there of them snapping and killing everything in the tank
good luck with choosing your new fishies, if you go with oscars you wont be dissapointed