Get a couple filters and put them on your 15 so they can mature. That will work better than using the water. The decorations, plants, filters will all have the bacteria you need to help cycle the new tank. Even if you don't want to permanently keep those decorations in the 75, put them in while you are cycling.
I always keep extra filters running on my tanks, so if I need to set up the Q tank the filter is ready (and therefore my tank is cycled).
Don't use your Kuhlies (or horseheads to cycle). They need a mature tank. I used Zebra Danios to cycle my tank, but that wasn't smart because they like cooler water, and I didn't know that.
Let us see your new tank.
