Pidgin's online help has some info
Your ~/.gtkrc-2.0 file may contain the text " -- THEME AUTO-WRITTEN DO NOT EDIT". That may apply only to the theme on the next line, not sure. In any case, changing gtk-font-name will change the font for all applications, and the link above gives some hints on how to set seperate fonts for some things.
This worked on light, and I don't know any reason why it wouldn't on SOHO. If the file's not there, try "locate gtkrc-2.0". If you don't find one somewhere (assuming locate's database is current), create ~/.gtkrc-2.0. Just having the gtk-font-name line will work, more can come later.