If you wanted to keep O. mercatoris dwarfs that are of equal size and have been raised together (found in LR together or tank hatched sibblings) it is viable in a 20ish tank and I think they are more active when kept in pairs. In a 50, however, you are not likely to see them but it would not be harmful. Obtaining them is a major undertaking as we have not seen many in the last few years.
All other common species (there are a lot of unknowns) will not survive together in something that small for two reasons. One, keeping the water clean, even with an equivalent sump is next to impossible and two, the likelyhood of one killing the other is almost guaranteed. O. vulgaris have been kept together with mostly successes (some predation is usually reported from the labs/aquariums that do this) but the tank must be much larger (roughly 1,000 gallons but you would need to contact some of the aquariums to get a comfortable minimum. I have seen a pair in something a bit smaller but was never able to get the exact gallonage when I inquired). Any attempts at keeping anything else has ended up poorly for one or both octos.