I just used a 23G 1" yesterday. Didn't hurt going in or anything. To much adrenaline flowing in my body lol. The only thing I have today is a very small black&blue and my bicep is huge!!!
I used a 22gauge 1" in my peak... Went in flawless (hurt alot less than the 20gauge 1" I used in my delts once-lol) It didnt hurt till the next day, when I couldnt move my arms... guess 1.5ml sus will do that.
I use a 'green' 21G 1.5 also with no problems. Obviously you don't stick it all the way in. I don't find any significant difference between 25g and 21g pain wise anyway other than it's easier to draw and inject.