Can one Mac boot up to several OS X installation? Since there will be iOS 6 / Xcode 4.5 Beta or Preview and Mountain Lion Beta or Preview, I wonder if using an iMac with 1TB of hard drive, can it be partitioned using its Disk Utility into 4 or 5 partitions, and install Lion on each of those partitions except one partition for Mountain Lion Beta?

Or alternatively, if it is a Mac Mini with 500GB and it is partitioned into 3 partitions?
You can definitely have any number of OS X installs on any number of partitions. Use the Startup Disk preference pane or hold down Option at boot to choose which partition to boot.