I looking for a carabiner that will fit...
I looking for a carabiner that will fit in the small whole one a petzl ascender. Will the black diamond mini pearabiner with a screwgate work in it?
The "Mini" refers to it being small for a pear shaped biner, according to BD.com it is "Slightly smaller than the RockLock", which is their largest biner. If you are set on a BD try the Positron. http://www.backcountry.com/outdoorgear/Black-Diamond-Positron-Screwgate-Carabiner/BLD0556M.html
BC has a unconditional money back guarantee, so even if you buy either one and it doesn't fit, you can send it back no questions asked.
P.S. BC is sold out of the Positron right now, so if you need it now... The quicksilver is more or less the same biner with a hook nose rather than the key.
I wanted to do this exact thing. The solution I came up with was to use a petzl mini-oval maillon rapide. The gap in the mini-maillon is about 16.5mm wide so it should be able to handle just about any rope/biner you want to connect to it.