I can't speak for the Zupreem, but if you do that one I'd get the "naturals" not the artificial colored ones. I think that just like any of the "good" brands, they are formulated to be nutritionally balanced.
I can definitely speak about Harrison's. All of my birds and past birds eat it. Various species including budgies. I know not everyone's birds like it, but I've never ever had a problem with anyone not liking it. I never really even needed to use a conversion method. I guess I got lucky!
I can definitely give you a testimonial about Harrison's with my beloved Red Bellied Parrot, Robin. I've had Robin for 23 years now, and he still loves his Harrison's. Early on, for years, that was all he ate. His blood work at the avian vet had always been within PERFECT parameters.
Another interesting thing about Harrison's is that I've noticed it gives birds the brightest most colorful plumage possible for their species.
I have fed all birds regardless of size, the small size (fine). One bag for everyone. I used to feed the super fine to the budgies and the Aussie keet species, but it's not always in stock at the store when I needed it. One day when I was out of super fine, I gave them all fine, and even the English Budgies were able to eat it. Even a standard budgie I had could eat it. Think of the pressure when a budgie bites you, even that tiny beak is way more than enough to chew the fine size! I think it's just a matter of them getting used to eating the fine size, not that they can't.
To answer your question on size, yes the super fine is like millet more or less.
Edit: you might be able to get samples before ordering a larger amount. I know at least Harrison's has a sample size. Contact them and see if they'd be willing to send you some if you pay for shipping