Eric Weddle is probably going to play as that SS/ILB combination player. At this point, he probably has LB speed but his strength has always been his ability to quickly read plays, and generally, he is a good tackler.
Since Morris plays a passive zone his lack of speed won't be noticed, i.e. Morris' scheme is wheelchair compatible.
Blake countess probably has around 4.6 speed at his age so he definitely can play in a zone scheme.
At this point in the season, they are grabbing vets who can get up to speed quickly having played in a variety of schemes. Both Countess and Weddle fit that requirement as both having played for Wade Phillips who also played a man/zone coverage scheme.