Depends on where you inject. 25g 1 inch works well for most places if you don't have high BF. I pin delts with 28-29g 1/2 inch slin pins all the time.
Not sure why anyone would tell you it won't work. Now I would use a 27-29 at 1 inch because it's flimsy but still hard to break. But it's not any problem to get the oil thru.
why would you want smaller than 25g? I can even feel the 25gs in by delts. Is the pain unbearable?