To check if FreeBSD 9.1 works fine on the iMac, you can use the live CD:
http://www.freebsd.org/doc/handbook/using-live-cd.html
The second part of your question I can only answer from logical thinking (no hands-on experience with MacOSX + other OS on the same HD, so you will assume all the risks): It should be posible to dual-boot Mac OS X and FreeBSD, just resize the Mac OS X partition, create FreeBSD slice, install FreeBSD in it, and instruct the FreeBSD loader to add Mac OS X to the menu...