You can do mate, it really increases the chance, and if you breed silver to silver for a few gens its almost 100% that they end up all f****d.
Micros can be the runt, but normally they have been selective bred, smallest to smallest for some time.
You can line breed anything, that's how different strains, breeds etc are created in a true breeding sense.
in my litter i have a kit that is no bigger than a new born ferret and there 3 wks old and shes got hair atm so, i think i might have a good ferret for rabbiting