I agree with all above to be fair. The HRP would be perfect they are a great alternative, equally their other relatives like Sajica, Cutteri and Nanoleutus would all work.
With Severums I have a rule of minimum 75g for Golds/Greens/Red Spots etc and 55g for Rotkiels and true Severus. Gold/Green Severums often get 10 inches long and in a 55 it does not give much turning room where as most 75 gallons have a 20 inch footprint which is going to let them turn more freely than just on the spot. But so far as your gold is concerned the 55 would be a good home for him/her for a short while but common Severums (Heros Efasticious) grow pretty quick I have a young pair of Heros sp.Belem and they have gone from around an inch each to three inches in maybe 2 months, the male is probably over 3 inches now.
So far as compatibility though a Severum would fit nice with your group as it is
But I would be tempted to swap the Gold for a Rotkiel
Wills