Start with adults. Four is the absolute minimum I would do, but six is best. Three fish opens the door to a couple of scenarios. A) One dominant fish picks on two submissive fish until the submissive fish get stressed out and sick. B) Two dominant fish pick on one submissive fish until it gets stressed and sick, then one of the dominant fish becomes the new submissive fish, etc. Discus pecking orders are a complex thing, but basically the more fish you have, the better chance that the aggression will be spread around more evenly.
When you say things like, "I want to keep discus with geos and severums," there is no definite answer. It really depends on the temperament of the severums and the type of Geos. A lot of discus keepers have orange heads with their discus. Smaller, more peaceful geos like G. parnaibae would work as well. I've also kept severums with my discus but that has mixed results. Sometimes you get a peaceful severum, and sometimes you get a severum that learns it can push the discus around (especially at feeding time) and they take advantage of that.
Another thing to consider is tank size. Six adult discus will do best in about 75 gallons. You could get away with 55 to 60 gallons if there are no other tankmates and you're doing large water changes, but a 75 would be best. Add in a group of geos and severums, and you'll need at least a 125 IMO, maybe even bigger. This is assuming you keep the geos in groups like the discus, which is usually how they fare best.
Adult discus need to be kept warm, 84F or so, no lower than 82F. This shouldn't be a problem. Most of the SA tanks in my house are kept at 82F. The most important thing you can do is make sure to provide them with adequate water changes (if the tank is moderately stocked you can get away with one big one a week) and make sure they're getting enough to eat. Discus tend to like to pick at their food and forage after feeding time. Geos and severums will not usually let food stick around long enough for that to happen.