Hi Sandyway,
on your skeg questions, let me take them in reverse order...your question about its impact on space, and then its impact on handling.
on space, you can do an install that has almost no impact on space to the extent that you keep the skeg away from the the hatch opening. in the picture Mark showed above, in the petrel, he has the skeg centered on the rear hatch which interferes with getting stuff in there. on the picture he has of the CLC 17 LT, the skeg is rear of the hatch opening. and i would imagine there is very little interference.
when i install skegs, my preference is to place it well behind the hatch opening and i have never had it practically interfere with space. as a retro-fit, it can be a bit trickier to get the skeg in well behind the hatch....but doable...just a bit more planning.
on impact on handling, i have a petrel, a frej, a shearwater 17, a nighheron and a clc17 and 14. i only have the skeg on the petrel, the nightheron and the frej....all of which have pretty rounded chines. on the shearwater and CLC hulls, which happen to be hard chined, they just tracked well enough in the conditions we paddled, that it never became an issue.
so my advice would simply be, if your boats are handling well and you are not making a lot of correcting strokes....then a skeg is a nice-to-have. alternatively, if you are finding it challeging to keep the boat tracking well, then a skeg, in my experience, makes a big difference. i would also note that how a boat handles is also very inter-related to who is doing the paddling and their weight vs the designers assumptions. a boat that tracks well with the right person who matches the weight the designer expected can be a very poor tracking boat if the person paddling it happens to be significantly different than those assumptions...particularly if a person is not heavy enought to have the boat on its design lines. so a skeg, as a feature on a kayak, can also improve the usability of a boat being used by different paddlers.
all that said, i have never really viewed this as a trade of space vs the skeg.....they should co-exist happily. i think of it as skeg complexity/hassle vs handling improvement.
on how to do an after-market skeg, i am a fan of Superior Kayaks aftermarket skeg. very reasonably priced and 100% fibreglass (vs plasic) doesn't take a lot of space....easy to install.

the picture above is my petrel with the superior kayak skeg extended...it's a bit hard to see in this picture, but when you open the rear hatch you have to put your head in the hatch and look aft to see the skeg box....it does not interfere at all with the space with regard to what i have ever tried to carry....the limiting factor for me has typically been the hatch size in terms of what I can get in there
h