I don't think the NCAA particularly cares whether the schools are rivals. They do allow testimony from anyone who has knowledge.

As for rivals, if you consider Alabama and Tennessee to be rivals, the NCAA allowed and used a LOT of testimony from Philip Fulmer and the UT booster guy who died recently (Roy Adams?) in making its case against Alabama in the Albert Means mess. They apparently didn't use any UT players, but a rival head coach is pretty much the same in terms of credibility, which I assume is what you're getting at.

Though we are not rivals by any means, the NCAA also allowed Lester Earl, who had transferred to Kansas, to testify against LSU in exchange for immunity back in the '90s case that resulted in severe sanctions for our basketball program. In his case, they literally told him that he would not be eligible to play unless he gave evidence that implicated LSU and that he would be eligible immediately if he did. If ever there was a "witness" with an incentive to lie, that was it. Earl later admitted this and that a lot of what he told them was a lie, although LSU was certainly guilty of enough to get hammered without adding whatever lies he told them. The point, however, is that the NCAA didn't much care that Earl had a strong incentive to make false allegations in the LSU case, so they may well not care that a current Miss. St. player may have one in this case as well. The fact that they gave him immunity in order to talk to him suggests they think he's probably credible.