We electrofished two central Texas ponds last weekend. One is 2.2 acres, the other is 4.8 acres. Both were built seven years ago. Neither were stocked. The owner wanted to stock both of them, but on a preliminary site visit, I noticed some small bass hanging out in shoreline weeds. I suggested we electrofish before stocking any fish. There's an upstream lake, across the highway.
It's a good thing we did.
Both of those lakes had mature populations of bass and bluegills, with a few adult bullheads. I've never seen such a good ending, in this case. During timely floods, just the right numbers of bass and bluegill migrated and established.
For this landowner, it meant he didn't have to spend money to reach his goals.
In your case, unwanted bass could be a problem. You better get them out before they spawn.
How'd they get in there? Reputable hatcheries can make a mistake, especially if you are buying fish off a truck. It's not hard to mix species. But, conscientious suppliers go to great lengths to keep from mixing species.

Teach a man to grow fish...
He can teach to catch fish...