Almost any salt will mix thoroughly after a few hours. Some people wait overnight some don not. I use reef crystals and if push comes to shove I can have a batch ready in 20 minutes. I prefer to let it sit a few hours but there have been times when i needed salted water asap, you want to be prepared in case you do need it quick. I have used a few other salts and while i like red sea for it's element content I do not like how it left residue in my mixing garbage can. lol
And yep, a mag float is like a necessity, at least to me. lol
As far as shrimp go by the way, they are all pretty good together except for coral banded shrimp, they can become nasty and kinda large. But yep, i have kept peppermints, cleaners and fire shrimp all together with no issues. A fire shrimp is another great idea, they are sooo red they stand out quite a bit.
I personally have neve4r had any issues with emerald crabs either, I love how they do their little dance on the front glass, it's awesome. But porcelain crabs are cool too. You might find sexy shrimp too tiny really to get much enjoyment out of, unless you don't have much going on in the tank. they are tiny! Nad for the record they do better in odd numbered groups for whatever reason. Go figure. hehe