Hi HanDiiMaNz,
It's not unusual for finches to travel for food in the winter, the classic case being the brambling that overwinters here in the UK, having crossed the North Sea in great numbers from Scandinavia.
Bullfinches and Hawfinches are only very rare visitors here in our part of NE Scotland, so I imagine that they too are just in transit, looking for richer pickings.
We do have a resident population of chaffinches, greenfinches and siskins who don't seem to travel too far afield though, as some of the individuals are well known to us (e.g. the 'Pope' chaffinch who sports a white 'skullcap'
) and have been here a number of years, all year.
In response to teknomage, we certainly don't seem to have aggressive siskins here ... they're all pretty amiable really ... it's the greenfinches who tend to be the pugnatious ones, but usually only with other greenfinches ... they tend to ignore chaffinches, goldfinches or siskins even when they are sharing the same feeder.
So I'm guessing maybe the agressive behaviour your are observing is possibly something they have acquired/learned, rather than being inherent in the species.