You can't use other fans than 80mm ones without modifying the fan holders, but I have seen many people in the forums, that have done that. Note, that you can't screw the fans directly to the case, there are no holes.
26 dBa sounds rather silent, you won't get much more silent fans without an effort, although I use 4 Pabst NGML 19cfm@12dBA 80mm fans, but I think they are difficult to get in the US.