No you cannot install Windows on a external drive by itself and use it with your Mac.
To setup up BootCamp for Windows 7, you need OS X installed on a external drive, then boot from it, then run BootCamp, the install Windows like Apple instructs. However I haven't seen this successfullly done, nor do I bother with Bootcamp preferring virtual machines instead.
You'll always have to run Windows from the external, because Windows is copy protected and takes the particular aspects of the hardware into that, can't have it on both drives at the same time, but you can transfer from the external to the internal with some jumping through of hoops via Redmond.
So i have a 1TB USB 3.0 external and i want to use it to install windows as i only have 30 gigs of free space on my 256gb ssd.
What you can do instead is reduce the personal content of your SSD down to what you will need for Windows 7 partiton, keeping that on the external drive.
Only you know what your plans are for Windows 7, if it's 3D gaming or high CPU uses, then BootCamp is the way to go.
You might decide a virtual machine (running any Windows or Linux) in OS X is a more preferred option.