Normally when you download stp file from suppliers, the assembly has no mate relationship. Instead of mating every single parts, can I RMB and select Fix?
Yes you can...in fact, you can highlight all the components in the feature tree then RMB and fix them all.
On a side note, parasolid assembly files come in with everything fixed instead of floating (iges and step assembly files come in with everything floating).
Some suppliers have actual SW parts but I'd do the step files most of the time and then look at what I had and make the decision to fix then.
Steve C