Discus are difficult, and if this is your first planted tank I would suggest avoiding them. It also doesn't help that discus prefer much warmer temperatures than most plants enjoy, so you might find yourself unable to keep certain plants if you went the discus route. I would Instead opt. to do angelfish.
I don't now much about the cat so I couldn't tell you how it would be with the bichir, but I might be concerned about the congo tetras with the sen. I would have the school of congo tetras go in before the sen ever touches the tank. As far as the tank being overstocked, take it form me that it could be a lot worse in a 75 lol.
I have 6 angels, 2 bichir, a pleco, a parrot, a pearl eartheater, a trio of rainbow cichlids, and 5 pearl gouramis. Now, I am pretty sure that over time I won't be keeping all of what I currently have (if the angels start pairing off I will end up adjusting their numbers accordingly, and I originally planned for the rainbows to be a pair but it seems that the 3 I have are not wanting any of that for one reason or another)
Just make sure that you have ample filtration, I run my tank with 2 eheim 2217's.
As far as plant care goes, I like ecco complete for my substrate, but if I were you and if this is a brand new tank, I would look into doing dirt substrate capped with either sand or ecoo complete. Then, decide upon what kind of plants you would like first, and buy a lighting fire to match the level of light intensity that you desire. If you are running anything more than low lights, I would tell you that you should get a powerhead for more water circulation and be dosing fertilizers after some time. If you end up running high lights, you will need a pressurized co2 system, that or deal with a ton of algae growth. I would also set the tank up with just plants for a while, and not add fish until all of your stem plants are rooted into the substrate. I can tell you from experience that it is NOT fun to have to replant a good fourth of your tank every other day :\
P.S. when you hit 5 posts, please PM me.