You should be able to do that, you would need to repartition your USB Drive so it has an MBR or GUID style partition setup, by default most drives dont come like that.
It can be done from OS X Disk Utility or using a third party tool like Parted Magic.
Dont know about the distros but they should not touch the harddrive bootloader if you are installing to a USB disk but I havent tested this. In doubt you can always disable the disk before doing the installation.