You can select the boot partition from early startup menu. If other version of OS needs reboot on first run, then it's easier to have it as bigger priority. Then you don't need to go early startup menu all the time (which works too)
Better keep the bootable partitions under 2GB of size and under 4GB area of the HD, at least with internal ide and standard filesystems.
Other way is to install separate operating systems on the same partition. You'll need to remove system assigns and assign them to other location. You can make custom boot menu for that. For example there are programs in aminet which bring menu up if mouse button is pressed during the boot etc.
