My old spray gun I used to use for spraying evercoat g-2 (2.2 tip needed) was an old non gravity feed gun from HF. No clue what actual size bit I used, but I slightly enlarged the nozzle opening & it sprayed the heavy coating ok. There was plenty of extra taper on the needle & I had no sealing problems. I twisted the uninstalled needle against the cap after drilling to help with seating.
Are you wanting the small gun to get into small places or just to clean out easier after only using a little product? I guess another option for a small gun would be reducing the product, but spraying more coats with proper flash times of course. You also may be able to get a smaller cup for your primer gun.
Only used a little spi epoxy so far, but I have poured extra back into a mixing cup, masking taped the top shut & put in a plastic bag in the house where it was cooler to re-use later after slightly reducing, taking some of the pain out of small spot priming while doing metal fab work. Had some sit almost 5 days , was fine. Had some sit 2 weeks, turned into a hockey puck.