In the old days, when we all did fish-in cycles, the advised stocking for fish was less than 25% of the final stocking level. At that time we also used the inch per gallon estimate fairly regularly. I have yet to see anywhere in your post how big the tank is so I can't begin to guess whether you have enough fish to perform a cycle. The nitrite readings look like you do not have much help coming from the previous cycling that may have been done. Ammonia being well controlled at the moment is a plus but if you never see any and don't see other chemistry moving, you may be a bit lightly stocked to conduct a fish-in cycle. Do not buy fish that you don't want to have in your tank. If you like the zebras, go ahead and use them to cycle the tank. If you don't care for them, I don't, then get something hardy that you do like. The first step is recognizing that every fish you add to the tank will mean more and bigger water changes than if they are not added. The next step is deciding if what you already have is enough fish considering the ultimate stocking levels and how close you already are. If you have a big tank and only have a 2% stocking level right now, more fish will be needed to do a fish in cycle. If you have a smaller tank and are at 15 or 20% stocking levels, there is no way you want to add more fish.