We've confirmed that Kensington two-button and four-button ADB mice here at Be work fine. We have found Mouse Systems and Logitec multi-button mice problematic.
In general, if your multi-button ADB mouse follows Apple's January 1994 Technical Note, ADB -- The Untold Story: Space Aliens Ate My Mouse, the BeOS will accept those multiple buttons. Most mice made after 1994 adhere to this Technical Note. Contact your multi-button mouse manufacturer to be sure.
A command-key equivalent is available for one-button mice (Control-Command for right-button, Control-Option for middle button). Additionally, the Tracker accepts click-and-hold as a right click for context-sensitive Tracker menus.