Tony,
I believe that cotoneasters are good trees for all levels of bonsai practice and am quite a fan myself.
In my experience these are very tough shrubs having orginated in dry mountain areas and able to tolerate dry exposed conditions.
Although your zone is colder than mine, hard pruning and bare rooting at appropriate intervals (i.e. don't do it all at once unless you're feeling bold) shouldn't hurt them.
Heres' a thread on two 5 " high shohin I'm developing from nursery stock.
http://forum.bonsaitalk.com/showthr...ght=cotoneaster
I also completely barerooted, root pruned a very large one on Saturday.
http://forum.bonsaitalk.com/showthread.php?t=17481
Three hours with a home made root hook.
Hope this helps.
TimR