Can fish like Guppies, Platies, Neon Tetras, Cherry Barbs, Harlequin Rasboras, and smaller nano fish (Celestial Pearl Danios/Chilli Rasboras) eat the whole Mysis Shrimp? I don't want bits floating around.
Not 100% sure on that as I've never fed those fish mysis. I used it back when I had my reef, I'd chop it up some to make it smaller bites. I'm sure if you did that they'd eat it no problem.
Cherry barbs wouldn't have any issues. Unsure of your tank plans, but I wouldn't add cherry barbs with any of the other fish you have listed. They tend to be pretty nasty bullies in most community tanks.
If the size is a concern though, both frozen brine shrimp & frozen mysis shrimp are usually readily available at most LFS's. So I'd just go with brine when you wanted to supplement some protien & such.
Neons and Harlequins tear mysis shrimp up! I've been trying it out for the last month and they go so nuts over it that I have to hand feed my gourami or they don't get any. I've actually seen a jealous neon come rip the shrimp right out of the gourami's mouth O.o
You shouldn't take life to seriously. You'll never get out alive.
I think mysis are pretty big.. they are more solid than brine shrimp if thats a more accurate word... after defrosting brine shrimp, the bodies are all soft and already falling apart and can squish pretty easily, while I find the mysis still holding its shape up well (like the photo above)
Fish like guppies, tetras will have no big issues but I can see nano fish having trouble swallowing one. A small rasbora or cpd, I could see taking little bites out of them.
I read that both frozen mysis and brine shrimp ( there is no "s" on the plural of shrimp ) (Sorry, one my pet peeves!) have 60+ % protein. So they are both good sources of nutrition. I find many fish don't favor mysis because the shell is so thick. Even my 4+ inch rainbows often spit it out. Plus it is often inconsistent in what size they are from one batch to the next from the same brand.