I have a 75 gal Discus community tank that I've been running for about 15 months now with IMO pretty good success, so I'll give some feedback.
1. Discus definitely do better with cleaner, fresher water, but if you have a very well-filtered, fairly heavily planted tank, you could probably get by okay with only a weekly water change. If you do that, you should definitely stock the tank with adult Discus from the start. It's pretty much proven that discus grow better with more frequent water changes. Sure, there's always someone that'll tell you they've bucked the trend and had success, but go with the odds on this one. Discus are too expensive to take crazy chances. You can feed adult Discus less often, therefore, less waste means you can get away with less water changes. I run two XP2's on my 75g, and they have to be cleaned monthly because the waste starts slowing the flow. If you don't care about your discus stunting, then get younger smaller ones, and only do weekly water changes. You'll end up with some of your discus only growing to 3 inches or so. Personally, I do 2-3 40-50% water changes each week. I have a fairly heavy bioload and I have younger Discus that get fed plenty. Lighter bioload with adult Discus and you could probably could get by just fine with a single 40-50% water change. Keep an eye on your filter flow, too!
2. With a Discus community tank, for tankmates, the main thing to consider is which fish can tolerate the higher temps (82-85 degrees, depending on your Discus' age and whether or not you're trying to breed them). I currently have Cardinals, Rummynose, Marbled Hatchets, Sterbai cories, Otos, a Bolivian Ram (just added), a Florida Flagfish, and an albino BN pleco. I have also kept Black Neons, Lemon tetras, and German Blue Rams without problems. For a corner tank, I think I would forego the Rummynose, because they seem to like a longer tank, because they are great schoolers and constantly swim from side to side on mine. A corner tank might seem a little cramped for them. Don't forget invertebrates since your tank is going to be planted. I have Amano shrimp, Ghost shrimp and Nerite snails in my Discus tank (and every other planted tank I have, for that matter). The Amanos and Nerites (along with the Otos) make it so I NEVER have to use an algae scraper. The Ghost shrimp are excellent scavengers that eat any leftover food and along with good filtration make it so I see next to no detritus on the sand.
3. IMO, the no substrate thing is mainly for breeding tanks and tanks that have extremely young Discus. Other than a quarantine tank, I'd have no tank before I'd have a barebottom tank. They have no visual appeal to me in the least, regardless of what fish are in there. No doubt barebottoms are easier to clean, but sand is pretty easy too (especially if you have a solid cleanup crew like cories and shrimp), as detritus can't get too far down like it can with gravel. And too much detritus will definitely not be good for the good water quality that Discus like. Keep in mind that Discus do darken up some with darker substrates. This is not a myth. It depends on the strain, though, so do your homework. Most Discus keepers recommend a lighter substrate like white sand if you want your Discus to look their absolute best. I have Flourite Black Sand in my Discus tank. There are a couple in my tank that regularly darken up in an attempt to blend in with the environment.
4. Keep whatever color strains you like. They do like to be kept in larger groups (I have 6) but I've never seen any behavior based on color patterns. In my tank, there has always been one Discus that is like the oddball that the others try to bully (they are cichlids, afterall). This is usually the smallest one. I have lost a couple like this. They end up isolated and stressed and end up succumbing to disease. A few months ago I intoduced a couple of new ones about 2.5 inches each. One of them the others never bother at all, but the other one the larger ones occasionally try to bully. I know this one is going to be okay, though, because it doesn't back down at all. It stays in the group, eats as much as the others, and takes no crap. Just watch out if one of them starts to get isolated and constantly stays away from the group.
Discus in a planted tank is very do-able, just do your research and know your goals. If you want a planted display tank, with fewer water changes, go with bigger Discus from the start. If you want to get younger, smaller (and much less expensive, also) Discus, you are going to have to invest in more frequent water changes for good growth. If you want to successfully breed them and raise fry, you're better off going with something like a 20gal high barebottom tank for each pair.