Yeah I think you would need to wash the yeast cake for sure, or you will have a color/flavor issue. It's easy to go light to dark on re-pitch but not so easy to go the other way around.
And as far as quantity goes, not much. You will have lots of yeast from the 5 gallon, and would need maybe 1/10 or less of that yeast for your 1 gallon batch. This is rather shoot from the hip, Mr Malty would be better.
Primary #1: Empty #2: Empty
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