I would just use the plugs as they are, as long as there are no burrs on, and they go in easily or even snugly.  If there is binding, then a  cleanup with a die carefully adjusted may be in order.  Slop in the threads is definately not good.   I didn't know that Taipan even made plugs.  Are they any good?  I have one of their motors and it is great.  Aluminum washers should be fine, they may not last as long as copper, but are disposable anyway.  I suppose the expansion would be more even or more similar than copper or????  It wouldn't be any worse anyway, and would be less expensive to make.