How large of a tank are you planning on putting these fish in? Barebottom or planted? How large are these fish you are purchasing? If they are juveniles you need to really have them in a barebottom tank because they need large water changes daily and it is so much easier to clean up all the excess food and waste.
Discus really arent that hard. They just need a lot of determination, care, and TLC. Contrary to the common belief you do not need to have a low Ph. Just keep it at what your LFS has it at. With young discus they need top quality food. Live, frozen, and dry. I suggest different brands of discus pellets, Frozen Hikari Bloodworms, Frozen Brine Shrimp, Live Black worms, Live Brine shrimp for now.
Young discus require large water changes and almost constant feedings so they can grow. They need very clean water, so with young fish atleast 80-85% water changes must be done every day. You must also wipe down the insides of the aquarium becuase detrius will often attach itself to them.
Keeping multiple strains of discus together is fine, it really doesnt matter, more of personal taste.
Telling sexes is very difficult. The only way you could would be to witness them spawning, and assuming you are going to buy young fish, you have a long way off.
I suggest a 55 gallon and having 6 fish to grow out. This should be bare bottom, and cleaned daily. Feed them heavily 4-5 times a day with healthy foods i mentioned above. The temperature must be kept around 82 F aswell because discus need higher than other fish. Get a few good powerfilters on the back, cycle it, and you should be set.
Are the discus's eyes you are planning on purchasing large in comparison to their body? If so these fish are stunted, and you should really not buy them.
I also suggest you look at these sites and research.
Simplydiscus.com
daah.com
Good luck,
Daniel
Aquaforums.com