I wouldn't waste the time with a retractable skeg on those two. If you can get comfortable leaning the boat with your hips as you paddle you can make either of those boats go anywhere you want. At least I can.

I was out paddling with Coy in his Firefly and we got caught in some strong wind. I took the direct path and I noticed he veered off in another direction. We got back to the dock and he was saying he was having trouble keeping it going where he wanted. I jumped in it and I could make it go anywhere I wanted no problem. Not to brag but I was totally impressed how well it behaved in the wind. After a little questioning I realized he didn't lean the boat as he paddled but tried to turn it by using the paddle only when a little lean with a little paddle input will change the direction of the boat very easily. It took me a while to learn this but now it is just second nature to me. Keeping the boat on track in high winds is no problem.

My advice would be to build it as designed and then if for some reason you can't get comfortable just ad a fixed skeg, trim it a little at a time till you find the correct size that suits you. One of the boats I paddle a lot had some issues and I ended up adding a skeg to it. I started too large and modified it 3 times I think till I hit a size that balanced tracking and turning like I wanted. After trying this I would never hesitate to add a skeg to boat to make it suit me and the way I want it to paddle. Even so, I don't think you will want one.