Im a skippy, so I dont talk in gallons, I cannot answer All you questions, but will give some insight for you... Oscars are only suitable with Oscars. Every tank is different, I have started cycling 3x tanks, and I have used media from an established tank for all 3x tanks and the last tank I started to cycle finished the cycling 1st before the others so there are NO hard and fast rules, EXCEPT for 'patience is the key' Please be aware if you put it on your desk, your desk may not support it, think of the weight -example 100 litre tank, holds 10 -20 kg gravel + decorations + the weight of the water approx 1kg for every litre, so you are looking at least 100kg for thr tank NOT including the glass itself.
Did you check the parameters of your tankwater? Were the survivors of your tank near the top? How long ago did you get the plecos? Did they show any signs of illness? May have just been a number of things again, I am picking at strws here as you have not provided enough info to give an exact cause or even to go down any path to advise what went wrong.
