Just finished giving these two a test run. In short, I love it. For more detailed thoughts:
1. I like that both characters have abilities that are both powerful and unique. Metal Knuckles, for example, isn't just a metal copy of Knuckles, with Metal Sonic's boost. He doesn't have the precise control of Knuckles' glide, and he can't climb freely. He instead has abilities which are similar in goals to Knuckles' abilities, but sacrificing precision for speed. Similarly, Tails Doll doesn't have free flight, but he can reach high areas, and he sacrifices his ability to spin for a nifty Polygon Barrier.
2. I like that Tails Doll's red gem changes colors when he collects a shield. Instead of the shield displaying around him, his gem absorbs their abilities. And you can use the shield abilities by holding jump and pressing spin, so it doesn't clash with Tails Doll's in-air ability.
...Mostly. It takes some getting used to, and I find myself accidentally using shield abilities when I don't mean to. Maybe there could be a work around for this? Maybe assign Tails Doll's in-air ability to one of the extra buttons instead?
3. I do find it odd that Super Metal Knuckles turns kind of a golden color. I'm guessing he takes more after Super Metal Sonic than Super Knuckles, but I was expecting him to glow pink.
4. Is Tails Doll supposed to be able to go Super? It doesn't seem like it. Maybe this was a conscious decision to balance the character or something?
5. Speaking of Supers, I think both could benefit from having Super abilities. Metal Knuckles doesn't have a mid-air spin ability, so maybe give him one in Super form? For Super Tails Doll, maybe allow him to float higher?
6. I forgot to test this, but can Tails Doll's Polygon Barrier bust breakable floors? If not, he might get stuck in some levels.
---------- Post added at 01:03 AM ---------- Previous post was at 12:54 AM ----------
After transforming, Knuckles doesn't gain the same height boost as Sonic when jumping. I think his jump height is actually the same as his base form, but I'm not entirely sure about that. What I do know is if you cheat him into Death Egg Zone in S3K, even in his super form he still can't beat the miniboss due to it requiring a higher jump height. However, Super Sonic and Super Knuckles have something else in common: Enhanced flight capabilities, though Knuckles' is less. Super Knuckles gains a boost to his gliding and climbing but still runs along the ground, while Super Sonic can outright fly (Though often only does so in boss fights. He does however represent this in normal gameplay via his running sprites.)
This gives the impression as though knuckles is drawing out less power. As much as he needs to complete whatever his current mission is more efficiently without overdoing it. Super Sonic on the other hand shows no such signs of holding back. His display of raw power is enormous, and they even updated his idle animation in Mania to represent this, with electric sparks coming off him occasionally.
Super Knuckles is the only character not to get enhanced jump height when Super, and that is by design. Knuckles' weak jump is the only thing keeping him from taking Sonic's path. That's all it is about. The developers just wanted to keep Knuckles to Knuckles' paths.
In game color pallets mean nothing in regards to canon, that's all just technical stuff that saves memory by working within pallet restrictions. The way it visually appears is much more important, and you can clearly see
Knuckles turning much more bright pink than
Mighty.
The implication here is that Knuckles has a much more powerful aura than Mighty, rather than his fur just becoming more luminous. The amount of glow coming off Knuckles is much more reminiscent of Super Sonic for a brief moment while Mighty never looks that bright.
I think you're reading too much into this. Knuckles glows brighter because he's a brighter shade of red than Mighty is. Knuckles' base form is practically pink in Mania already. The pattern shown to us in S3&K, Mania, and other games where characters other than Sonic can go Super, is that hedgehogs glow yellow and everyone else glows a brighter shade of their original color. Tails and Ray shine yellow, Knuckles and Mighty shine pink.
Also, didn't you just say that Knuckles' jump height was evidence that Super Knuckles wasn't as powerful as Super Sonic? Super Mighty has an increased jump height, so by your logic, Super Mighty would be more powerful than Super Knuckles.