We're faced with the same situation at the Olcott Pen Project. We have 3 pens now and currently pen rear 50,000 kings and 3500 steelhead, but would like to add one more king pen since we are losing the caledonia fish.
My personal belief at Olcott is, though it's hard to believe, up until 2 years ago Olcott NEVER got steelhead plants (I do believe domestic rainbows were stocked there but no washington strain steelhead). That being the case, without an additonal pen for another 25,000 kings, I'd rather see that 3rd existing pen used for the additional 25,000 kings than 3500 steelhead.
Tim