Weird. I had ACL surgery last summer and I was told NOT to kick with a straight leg as it puts downward and upward pressure on the knee (after surgery). In fact, I was diving both pre-and post-surgery and the only kick that didn't hurt was the frog kick as it uses the hip and ankle joint, and outer thigh muscle, but the knee is isolated. When I would forget and scissor kick, my knee let me know in a hurry that I was kicking wrong! It's a year later and I don't use a straight leg kick at any time and have absolutely no strain on my knee while diving. I never switched my fins at any time (OMS Slipstreams) but did find that having the spring straps helped a great deal when having to bend your knee to put on the fins.
Just something to think about. Explain to your PT the mechanics of a frog kick and see if he agrees once he understands about the joint movement. It may not be that you need a different fin but rather a different kick.
Also, whenever possible, try gearing up in the water to lessen the weight and ask for help with your tanks. Hang in there. The first couple of months after surgery are tough but it gets much better if you stay with physical therapy as long as possible and push yourself to improve.
One last point: Not sure how long it's been since you had the surgery but you MAY have a slight increased risk of getting a DCS hit, especially if you're going deep. You may want to check with DAN about diving after surgery. There is a LOT of info about the risks involved.
Stay safe and have fun!