Inch rule
Well, the inch rule. Some aquarists suggest that you can add 1 inch of fish per gallon of water. Again, this depends on the surface area of your tank. Example. If your tank is tall without a lot of surface area to allow for gas exchange, this rule would not apply. Lets forget about the surface area for a second. Cardinals are about an inch long. If you want 20 of them, that would be 20 inches. Now since your tank is a 45, you still have 25 inches. Remember to measure your discus for length and height, because the are larger than the average fish. so if your discus are about 2 inches each, that would be 4 inches per discus. That would be 24. The cory cats are probably about an inch, so that would be 4 inches. Already you are over that rule. But if your filtration is good and you do water changes, I see no problem with it. Basically the inch rule is a good basis. You would be close, so I think you will be fine. Just remember that the discus will get bigger!!!Hope this helps!
45-20=25
25-24=1
1-4=-3
And how ever big your pleco is. You are close!