I think a 75g is just fine for a pair of severums. Not all species reach the same sizes ime, i believe liberifer are among the smaller heros and red tiger among the largest. Then theres heroina cichlids which are known as dwarf severums. All of these species are among the more rare imports where im at though, could be hard to acquire depending on your market.
While theyre not inactive by any means, i consider severums to be on the lazier side of the cichlid spectrum. Even in large tanks i usually just see them sort of floating around the middle of the tank if theyre not feeding or breeding. Not really skittish either. So while a 75g is not exactly roomy, i would consider it just "adequate".
EBAs go great with severums. The only problem i could see is that if you do have a breeding pair of severums, the big male will most likely play hell on the EBAs, killing them or driving them into one corner of that small tank constantly.
I think for a tank that size, 1 severum and a group of EBA would be very fun. If you happen to grow a big male severum, consider rehoming it or upgrading the tank if you feel it is too cramped.