Personally, I wouldn't have any problems going from a bigger beer to a smaller one with the same yeast (washed, or just dumped on the cake). You may have a bit more yeast than you need, but this hasn't been a problem in my experience. Generally, yes the trend is to start with the smaller beer so you have more yeast for a bigger beer but I don't see any problems with doing just the opposite. The important thing for me is just having enough yeast. I often use the same yeast for 6-10 batches in a row, and they don't all follow the smaller beer --> larger beer trend. I hope this helps. Best of luck...you'll love all-grain!
I could murder a pint! -Lovejoy