What about a series of 1 gallon pressure pots? You could set a bunch of them up for cheaper than buying a single piston style pump.
Run a line from your compressor through whatever pre filters/ drier filters you plan to use (if any). Then into an adjustable regulator to drop the pressure to the pots. (I say this because this is how I had to do it as my compressor puts out 175 psi and those pots are only good for 90 psi). Then into a manifold with quick connects. From there you split to each pressure pot which will have it's own adjustable regulator to control the gun pressure based on what you're spraying.
Into each pressure pot you drop a plastic 1 gallon bucket that you mixed your finish in if you need to catalyze or if not, put the can in the pot and start spraying.
This way you can have each pot labeled so you only run compatible finishes through it so there is no contamination.
I did this style set up because my main spray pump is a Wagner 40-15s that I only shoot CV, lacquer, 2k and other compatible finishes from.
I use one pressure pot to spray metal finishes from. One is used to spray stain. One is used to spray water based clear coat. Another is used to spray Advanced water based paint.
This way I don't have to worry about left over pigment messing up a clear coat finish etc. Even though I clean the guns/lines/pots well.