I am trying color flow for the first time. The cake is for tomorrow night. My decorations aren't that big. Will they have time to dry? Will the buttercream on the cake make them fall apart?
If humidity is high where you're at, they'll take longer to dry. In any case, 24 hours should be sufficient. Be sure to make extras to allow for breakage.
Piping onto plastic wrap will make the pieces much easier to remove than if you do it on wax paper.
Since you're putting the pieces on the cake so close to serving time, don't worry about interaction with buttercream. If there's going to be a problem, it won't show up for many hours. Just remember, if you're going to 'glue' the pieces to the cake, use colorflow or royal icing as your glue - not buttercream.
Yes, I find 24 hours is an OK dry time, depending on where you live. Longer would be better, but if you don't have time, you don't have it. I have never had a problem with waxed paper. My color flow pieces have always popped right off it. However, you may find plastic wrap or parchment paper work better for you. I have placed many color flow pieces on BC with no problems. It takes a long time for BC to break down a color flow piece - hours if not a day or so.