have to agree with saz, you made two or three threads in different sections about the same thing (adding the cardinals and getting them so cheap) you asked was it a good idea, you asked would it be ok, everybody said the same, no. its not a good idea, it will overload your filter, it is too many to add at once and sorry mate, you dont even have a test kit to test your water stats after adding so many fish??
i would go diwn right now to a pet store and buy a liquid water test kit. test your water morning and night, in the next few days your ammonia and nitrite will be rising. it wont happen immediatly as the fish need to poo a bit first!
test 2 x a day, any sign of ammonia nitrite do immediate water changes to bring it back down as close to 0. you have very sensitive fish in that tank (discus and the cardinals) if you dont keep an eye on the water stats and catch any spikes in time, you will either lose your fish or severely damage them