They are all stunted if they have been together for 4-5 years. If that is true, that big sev isn't getting any bigger.
Having a group of 5+ sevs will help spread aggression around so no one takes all the abuse. However, that is only recommended for sevs in tanks that are 150+ gallons (previous poster said he did 6 rotkeils in a 135, but rotkeils don't get as big as other sevs). In your 75, 5-6 severums would fight until only 1 or two are left alive.
I see no issue with keeping the big sev, even though it probably is stunted. Since it is passive and takes food from you directly, keep it. Rehome the other sevs (at their stunted size, you could easily get them a small 29 gallon tank and keep them by themselves if you are attached to them). Get a rotkeils sev to put in the 75 with the big sev you have. You might even be able to squeeze two rotkeils in there, but you would need to keep a close eye on aggression and rehome someone if things get too rough. Male or female haven't made much difference in my experience. Males only tend to be bullies when it comes to pairing off with a female for the fist time. Otherwise both genders tend to have the same range of personalities and attitudes. Make sure there is plenty of plants and/or driftwood breaking up the line of sight to help reduce any aggression that does occur. Also add some dither fish to help give them targets to release their aggression out on. My personal favorites are the lemon tetras. Just make sure it is something the sevs can't swallow. Get 8-10 of them and you will be all set. The bottom would be open to anything that doesn't get too big or aggressive. A group of 8-12 cories would work. I would pass on getting four small geos if you go with three sevs, but you could still do them if you stick with two sevs. A single pleco or some small (4-6") catfish will also work.
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