I disagree with only including some of the midbosses in this. I would prefer to show consistency with that feature, either have them all in or not at all. They can all be reached easily enough in Stage Practice anyway, even Stage 5 Nazrin. This is a Boss Rush mode, but it seems silly to include only some of the midbosses based on such a subjective judgment of what constitutes a "good" midboss.
Subjective? Definitely. Arbitrary? Nope. As Naut said, originally all the midbosses were left out. I did add Nazrin because I find she's the most fun out of the midbosses, but there are some other reasons I didn't put others in at the same time. I did take them into consideration.
The main issues are time, content and flow. I want to remove as much as possible that keeps the player from the meat i.e. the boss fights. There's actually a 4-second pause before each boss that I'm unsure about keeping because it's just dead air, and at some point I was considering just throwing all the bosses into stage 4 in a "true" rush fashion. While the midbosses of course count as meat, they're still separate from the bosses and in general don't stand well on their own, both pattern-wise and as a good use of time. Having stage 1 mid-Naz would really hamper every new credit you put in; Nazrin herself is already almost a chore to go through before Kogasa. Mid-Kogasa's patterns aren't anything in need of practicing (tap tap so complex) and is uhhhh not very good, so I just really don't feel comfortable with her there. Subjective call but I'd rather not bother. Mid-Ichirin is the odd one out because she's pretty ok, but I guess I just find having no midbosses then a midboss then none then another is even more picky than it is now lol. I would also have to get her to stay on screen and switch to boss mode right after. Lazymode.
Nueball I had to leave out mostly for technical reasons. From what I could understand, she's well-integrated into the stage and is way more than just an enemy call. I tried to look around for a bit, but it was more complicated than I was willing to put up with changing. This affects two stages and was part of the reason I originally just kept them all out. S5 Nazrin is not only much more challenging (and fun) than the other midbosses, but both of her patterns are two of the harder parts of the game. I had to put her in if only for that. Those two patterns also (quite subjectively) stand together more as an entire appearance, doesn't feel as much of "hi bye here's the boss", I guess. Anyways, I'm not saying keeping one midboss isn't subjective, but there are still reasons I didn't put in any others.
What about players making jumps from lower difficulties to higher ones? They would appreciate being able to get used to Nazrin's Busy Rod or midboss-Ichirin's faster lasers on Hard, or having a better chance to finally figure out the trick to capturing both of midboss-Nue's appearances.
Players making jumps from difficulty to difficulty shouldn't be practicing bosses or midbosses alone. It's more useful when you already have a general hold on the whole game and want to practice the latter half of a stage without going through the former. I don't think the stages are ever such that people should stop practicing them unless they have the whole thing down, including midbosses. The whole thing is designed so players play what they already know rather than introducing them to something unfamiliar.