On the other hand, if you get a SATA->USB adapter, it might allow you to boot. Depending if your BIOS supports that, of course.
As for
>>5,
well no, it does not need THAT much space. However all the Windows updates and shit will cause it to grow considerably after a fresh installation. Also, even if you install programs on another partition, many put files in the Windows directory. The registry grows as well. And so on and so on. And you also need some buffer, in my experience you don't want a file system to be more full than ~70% for performance reasons, especially with an operating system on it.
The risks of resizing partitions... well, a lot of data usually gets moved around physically on the hard drive. In the case of a power outage or if the software simply screws up, you might have to deal with data loss or even with one or more corrupt partitions. That said, I have not encountered this problem yet, but I still try to avoid having to resize partitions. Even in the best case it takes a lot of time because the data has to be moved.