you don't have to replace your filter pads on your emperor that often. the manufacturers suggest that you replace them every month because the carbon only works for a short amount of time before they lose their function as a chemical media. you don't actually need carbon unless you're trying to take out chemicals like meds that you put in your tank. you can just rinse your filter pads really well and then put them back in. your nitrite spike might have been caused by removing your pads because benificial bacterias can live on them as well. although i'm not sure if that was really the case.
and to start up your 10 gal tank you can use some media from your 90gal to help seed it instead of buying bio-spira. bio-spira is essentially like buying beneficial bacteria which you already have in your 90 gal.
The Nitrites were high before replacing any of the original carbon filters, and the one that was in there I just through away (didn't swirl it in the tank or rinse and reuse - just tossed it).
Now you bring up a good point for the 10g. I'm going to get another Emperor 400 for it, and would simply taking 1 of the current filters in my 90g and putting it in there do the trick? Or is there another approach you are describing?