not knowing if the main tank was ever treated with any copper based medications , makes it hard to determine if its worth using or not using , that's a risk you would need to decide .
anyway you look at it you are going to have to do a cycle weather its with the rock you have taking that unknown risk since you know all the previous owners fish just died off from unknown causes and were they treated with copper or clean base rock not having any of those issues, all rock will become live over time ,
It's hard to give you rock solid advice not knowing what the fish died from and if copper was ever used to treat ,
that call would be left on you , if it were me I would contact the previous owner and ask these questions , that's the only way you would know if you can proceed with the rock you have or to just replace it with base rock,
you need to realize SW
is not a cheap hobby and to risk loosing fish from the unknown
is it worth that risk or start clean ,
maybe some one else will chime in with there thoughts with this ,