Melissa-
 
If you are talking about the vellux like MY vellux blankets, I wouldn't
use it for hammies and sleep sacks.  My blankets are basically velvety
flocking on either side of a thin layer of foam.  If your ferrets scratch
the hammocks or sack, the flocking might come off and the foam is then
available for them to chew on and possibly form a blockage.
 
Melodyt and the terrific ten.
[Posted in FML issue 3895]