I agree with the notion that certain top stages are more suitable CPs than several middle row stages (PS1 and Skyloft especially). For size, Metal Cavern really isn't much smaller than Green Hill Zone or Yoshi's, and if I recall correctly the blast zones are bigger as well... I do hear people complain about the sloped floor often, though. Skyloft, however, has a more drastic slope in the floor and a layout that makes recovering easier, gimping harder and neutral play somewhat abstract, not to mention blast zones that are very close to the ledges despite the stage being fairly big. I feel like if Skyloft is qualified to be a CP, Metal Cavern should be too, haha.
As for my thoughts on the other top row stages, Halberd has stage hazards, Norfair has some clear issues as well as having a layout that makes gimping hard and gives certain characters silly options (such as Luigi with his up-b), and the other stages kinda feel redundant to me with similar alternative stages that are already legal. Drac's is huge like dreamland only with walls that assist in recovery and a less comfortable, changing platform layout; Rumble Falls encourages stalling and makes killing off the top a bit ridiculous with some characters due to the platform layout; Castle Siege is very similar to Skyloft at the top, the transition can save/kill people and is glitchy, and I just don't personally agree with the rotating platform at the bottom. The Pikmin stage is the only one I could see being suitable, really, but even that I don't think is necessary as it's similar to a bigger PS2. It could see it being argued as a decent PS1 replacement.