You can catch all the skipjacks you want at what I like to call the mouth of the discharge. It's the area where the canal first begins to spread out onto the adjacent flats. Sometimes they will be all over the flats on both sides (easy days), other times they will concentrate around the channel (14 feet deep and @ 40 feet wide) and you will have to let your rig sink to near the bottom to catch them (hard days).
For the cats I have always caught way more and bigger cats in the main river channel - not the steamplant canal. Thats not to say I haven't caught them in there because I have, but I have fished the place enough to know that most of them are smaller than what I'm after and you really can't (get on them) in there like you can out in the river. They are on the move if they are in there and you have to wait on one to comes by whereas out in the main channel I've found lots of "holes" where it becomes difficult to fish with more than 1 rod at times. Old Hickory lake is full of channel cats. Plus I like to avoid the idiot crowds and the spinnerbait/worm slingers so I generally catch my bait and get the hell out of there. Hope this helps.