I would say a 4ft tank is the minimum for a severum. They will reach the 8-10" at full adulthood.
Groups of severums are going to be hit or miss. I have seen 4 severums in a 75g without issue and I have tried to put 3 together and one of them constantly harassed the other 2. That being said, overall they are a fairly peaceful fish. Especially with other species.
Other South American cichlids that are generally compatible.
Satanoperca Leucosticta
Geophagus winemilleri
Geophagus dicrozoster
Geophagus sp. "Red Head Tapajos"
Geophagus steindachneri
Geophagus abalios
Geophagus surinamensis
Aequidens metae
Uaru amphiacanthoides
Biotodoma species
Mesonauta species(Festivum)
Mikrogeophagus Altispinosus(Bolivian Ram)
Hypselecara temporalis(Chocolate cichlid)
Apstiogramma species
Apistogrammoides pucallpaensis
Laetacara dorsigera
Laetacara curviceps
Acarichthys heckelii
Other fish
Most tetras that aren't snack sized. (I have not seem them predate tetras in my aquaria but it has been reported)
Most loricariids(plecos)
Corydoras species
Basically, most peaceful fish can be tank mates for a severum.
EDIT: A couple of additional things.
1 - Apparently I made a giant list of peaceful SA fish while I wasn't paying attention.
2 - While I believe all those fish are compatable with the Heros species(Severum), I was not recommending you keep them all and you should ensure you have a large enough tank for whatever additional species you would like to keep.