True....cycle does not work...you would be better off putting filter media from your established tank into the 75 to help it catch up but even then it probably not going to handle the bioload of that many discus right away.
You are experiencing a mini cycle since the discus put a much larger bio load on the system than your guppies and tetras. It will take a while for the benficial bacteria to catch up.
I would add them back into the 75 slowly, one at a time as the bacteria has a chance to catch up. And ANYTIME the ammonia is above .25 then do a water change of at least 25% even if it is everyday.
Test the 10 gallon too, even though it was established longer it might also be struggling to keep up with the bioload. Keep checking it because you might see a cycle there as well.
If you notice there is another cycle happening in the 10 thn I would put them all back in the 75 and do 1-2 water changes per day until the ammonia and nitrite are showing zero.
I had three discus in a 10 gallon for a short time and even though it had an established filter I had to do 3 50% water chnges a day to keep up on the load.
It might be pretty exhausting for the next month with daily water changes but it will eventually settle out.
I wish you all the best. Let us know if you have any other questions.