The bottles will/should have the thinning medium to use listed on it. The Floquil/Pollyscale paints are water based and suggest a 10% thinning ratio with water.
acrylic paints need water or acrylic thinner as they are water based.
enamel's need paint thinner as they are oil based.
I got my Polly scale US Med green from the Testors site and the paints are 1/2 oz and 1oz. I don't recall ever seeing 1/4oz in the Floquil/Polly Scale only the Testors paints.
You'll pay any where from $2.50-3.99 for a single jar of paint from the hobby shop, I have 3 hobby shops here that carry the aints and differ only .25-.50 from each other.
I had to go online to get the Polly scale US Med green 1/2oz bottles as no hobby shop here carried it.