I really suggest you go over to simplydiscus.com and read up a bit. Not caring about having perfect show quality discus is one thing, but stunting discus is cruel and harmful to the fish. It is not impossible to keep discus in a planted tank, and if you have adult discus, it isn't particularly difficult. However, juvenile discus require pristine water conditions and a lot of food. These are conflicting requirements. Young discus won't grow very well on flake food (if they eat it at all), and all discus should really be fed on beef heart and black worms. These are high quality, but extremely messy foods that will foul up the water quickly, especially when you feed them 6 or 7 times a day. This is why discus have a reputation as being hard to keep fish. Because juvenile discus have high food requirements, you need to change the water frequently (as stated before, in a 29 gallon, you would need to do something like a 50% water change at least once a day, if not more). In addition, discus have space requirement unrelated to the water quality. I would recommend you have at least 10 gallons of space for each discus. Keeping discus alive is not hard as people make it out to be. Growing them out to the size of dinner plates is. Please, for the sake of the discus, use the 45 gallon tank, not the 29.
I would really recommend you do some reading on both discus and planted tanks before you attempt this. There are people who have been keeping discus and planted tanks for many years separately who still have not mastered this combination. For starters, I would recommend you study light, co2, macro and micro fertilizers. Start here: http://www.rexgrigg.com
Nothing alone keeps algae away. It is the right combination of light, co2, and nutrients that allows plants to use everything better than and before algae.
As for CO2, you can have it work with your AC. Keep the water topped off, high enough so that there is no splashing on the AC return. More importantly, you need a better diffusion method than just putting it into the intake.
I am surprised no one has mentioned it before, but you need to get rid of those clown loaches. There is absolutely no way that they will work in a 29 gallon except for temporarily. They grow to be 15". The cories are a good idea, as they clean up food quite well. I would be wary of adding cardinals in with your juveniles. Young discus are not particularly aggressive eaters, so you want to watch them and make sure they are getting enough to eat, and not having the cardinals beat them to it.
Plants are not a bad addition to a discus tank at all, it is the substrate that puts discus keepers off. Discus keepers prefer bare bottom tanks because it allows them to clean the excess messy food out during the water changes easily.
I'm sure you wouldn't want to mistreat your fish, so just look into it a little more