I don't think there should be any problems with the two barb species other than the golden barbs liking their water a bit cooler than most tropicals. Their max temp is 24 deg. If you aimed for the 23/24 mark, the other fish you want should be OK. Or perhaps look at something like the 5 band barb which are orangey though maybe not to your taste. Don't confuse these with the similar looking, nasty tiger barbs
I don't think there is a preferred m/f ratio with barbs. Having both sexes will make the males more colourful but they aren't like livebearer males, pestering the females constantly. If anyone who has had these fish knows different, do let us know
Otos need a tank that has algae growing. Leave those till last. And be very picky about buying them. The ideal is to wait till the shop has had them in a couple of weeks so that the iffy ones have already died when you get them. They are wild caught by poisoning the streams where they live. Lots don't make it past this stage. Those that do aren't fed properly and they have an odd quirk that once they starve past a certain point they can't start eating again. Besides waiting for the iffy ones to die in the shop, chose individuals with plump bellies as those are eating fine.
And I don't see why you can't have both cories and otos. They live in different areas of the tank and eat different things. Don't be fooled by the term catfish when applied to cories - they are not vegetarian. They need a fair amount of meaty things in their diet and the best food is a sinking pellet that isn't all algae. Pre-soaking pellets makes them sink faster so the barbs, tetras etc won't get them all before they hit the floor of the tank.
As for buying more fish, it is safe to add around a third of the amount of fish already in the tank. That's body mass, so one platy equals about two tetras. You have enough bacteria to cope with the waste from the fish there now. Adding another third won't overwhelm the bacteria and they will very quickly multiply to cope with that amount of new fish. Check your ammonia and nitrite levels after each addition; you can get more when you've had double zeros for a week. If you stick to the one third amounts, you shouldn't see either of them showing up so you can add fish at weekly intervals.