Sorry to hear you are having problems wanda.. I have just a few questions, which are for you and for everyone looking at this thread. I am nearing the end of my fishless cycle, and so i am quite interested as to what has happened here.
I am inexperienced in doing this, but from what i have seen and read, personally, i dont think it is in connection with the amount of fish you have added, but my first thought is that perhaps you added the fish with too bigger gap. I think the bacteria dies off pretty quickly. You added some fish the first 24 hours, then added 3 plecs 24 hours later. I would have thought that in between the guppies and the plecs, the bacteria may have died off a fair bit.. Then, when you added 3 plecs, which i have been told provide a fair amount of waste, then this may have caused a mini-cycle to take place. I beleive this, because on friday evening i tested my tank and the ammonia was at 0 and the nitrites about 0.25, but then forgot to add more ammonia. so on saturday morning, performed the test, and they were both at 0, realised what i had done, then added the ammonia again. This has casued my nitrites to rise again (not ammonia) slightly when testing it on saturday evening, and they are still working their way back down to where they were.
This shows how a short period of time without the bacteria being fed the same amount can affect the nitrite reducing bacteria levels.
i would like some feedback on this comment to see if others would agree.
Also, what filter do you have. I will have to be careful that when i do my big water change, that i can do this as quickly as possible to ensure that i do not affect the bacteria. I shall probably remove the sponge holders and put them in tank water whilst i do this. You have a huge tank, and so this would be a bigger task for you.
I would follow the others advice and treat this as a mini-cycle.