I add flow enhancer to every mix I use no matter how thick or thin. However, to airbrush acrylics they need to be very thinned. The problem is too much water raises the surface tension and reduces the paint's ability to adhere to the model. For airbrushing I add flow enhancer, but I swtch to thinning with Liquitex Airbrush Medium. You can make the paint as thin as you want. If you don't mind the slightly higher expense of of the airbrush medium over the 50/50 future/water mix, you can use the airbrush medium all the time.
On a side note, Liquitex makes all kinds of mediums for acrylics. You can get gloss, matt, crackle, texture, thickeners, blending mediums and more.