I am employed in a new shop and I am seeing some of the other bodymen add fiberglass resin to the filler.This mixture is used for all coats. They say its thins it out and reduces pin holes. I have never seen or heard this before does anyone have input on this?

Some guys do it and some dont. But that is exactly what they are doing, thinning it out. I personally dont. You can use resin, and there are products that are made just for that purpose. One thing you can do if your just starting out is use glazing putty over your plastic as a finish coat. It works very good.

In the evercoat line of fillers you can add just about any product together to custom mix your own filler thickness.

For example you can add metal glaze to Rage to make a thinner Rage, making it spread a tad easier and sand better as well. The products in the metalworks "line" are all compatible, which include rage, rage gold, zgrip, xtreme, metal glaze, glaze coat, easy sand and euro soft.
Te tech sheets for evercoat even have a section showing you the steps to do this.

Evercoat makes a product called Plastic Honey that you add to "thin" out other fillers. But in my opinion its not needed.
All plastic filler jobs are finished off with a coat of poly glaze. The glaze coat sands like a dream, feathers out nicer and less prone to pinholes. I use 80 grit to cut the top of the filler then use 180 to finish sanding ALL filler.