Hi Seņor Discus, if you are asking why start with larger discus you have not done enough homework. While I know you have stated that you have kept a variety of other fish discus are different. In your opening post I see several beginner mistakes. Before adding plants, community fish and please, do not add snails (carry parasites) just keep a group of six or more discus 4 inches or larger. Get your feet wet with larger discus as they are way more forgiving.
If you decide to start with smaller discus raise them in a BB tank with large daily water changes. They need plenty of fresh water and food to grow properly. We have had many beginners come here trying to raise discus under the conditions in you describe in your first post and run into a variety of issues. Issues include sick and/or stunted discus.
Please read the stickies in the beginner section of the forum, here's a few to start with;
http://forum.simplydiscus.com/showth...CH-THIS-VIDEO!
http://forum.simplydiscus.com/showth...hat-not-to-buy
http://forum.simplydiscus.com/showth...ed-with-Discus
There are other stickies also worth reading. Please check them out.
Pat