I use 25s all the time and dont even warm them up, if you cant push oil through a 25g pin you need to start working those forearms, seriously. It is better to go in slow than squirt it all in one spot too quickly.
I have used a 27g but they are only 1/2" so I would prefer to use 5/8" 25g. When I use the 27s I go in my delts, they would probably be great for biceps for those who do those shots.