So I played Flow with me and Bumper (or fLOW With ME and Omper if we're going by the logo) and I thought it was pretty cool. It does a good job of introducing its gimmicks, but falters when it comes to the execution and combination of these gimmicks.
For one, the difficulty curve is all over the place, which may largely be prescribed to the super claustrophobic hallways in which the level is set. The player has very little room to maneuver and some setups make me wonder whether or not the level was ever intended to be played as big Mario. The second jump of the second section, however, is ridiculously tight when playing as small Mario. I imagine you never got hit in this section after collecting the checkpoint and, as a result, never had to go through the jump as small Mario. Even if you argue that it's doable, it's certainly a stark contrast with the rather mild first section.
The sections that follow have some cool setups, like the spinjumping on bouncing spinies. Unfortunately, it feels like there is so much untapped potential here. The level introduces an idea, uses it in one setup and maybe another two later in the level, but it doesn't really see any of them through to a point where they become really interesting. On another more architectural note, as the level went on it became more and more open, which I consider a good thing given the nature of its gimmicks. Despite this, however, the issue of rather claustrophobic setups came into play once more. Take this setup, for example: it's nigh impossible as big Mario since duck-jumping is not an option.
Then there's the autoscroller. It's unfortunately on the short side and unrelenting in its opening seconds- the player starts out far too low and immediately loses sight of the floor they're standing on. It's also a rude introduction to the somewhat clunky SMB3 spinny platforms. The section which introduces them is quite charming- I didn't notice it at first and was sent back flying. To throw them at the player over a bottomless pit in a ruthlessly swiftly beginning autoscroller is a little much, though.
One final comment: where the coins at? I understand that placing them is by no means a necessity and more than anything convention, but I think that coin trails are actually immensely helpful at guiding the player's trajectory in tricky jumps, of which this level has a few.
In conclusion, you've basically got the first few steps down- the introductions to your gimmicks are really good and the individual setups which expand on them are too. However, the overall difficulty curve of the level and the final synthesis of all your ideas can use some work.