Generally agree with comments above, with one exception. I bred red heads for a while and had no trouble keeping them in a 75, whether as a group, a pair, or a pair with an extra female or two... in various combinations of tankmates, including a severum. So imo the problem isn't keeping geo Tapajos in a 75 gal tank.
But-- whatever history or dynamics led to your dominant male becoming aggressive, there's not much you can do but remove one of the males at this point (or temporarily divide the tank until you can sell or trade him). Adding some juvies to rebuild a group might be okay, assuming your aggressive male settles down without a single rival male in the tank, or adding several other adults could work by completely changing the dynamics-- but in your scenario, once one of your males targets the other, there's little chance it's a problem that will go away without some intervention.
The other thing to keep an eye on if you have just the sev and geo (and plecos) for a while is that the severum doesn't decide he doesn't like the geo around. Generally sevs and geos get along well, pretty much always ime and normally I wouldn't expect a problem, but just the two cichlids in the tank is a little bit of a wild card with your male geo's attitude. May not be any problem, but just something to watch.