^^^ The different pieces wouldn't be a problem. If you put something heavy on top of the poster, it will flatten it out for the best possible scan.
What do you mean by different colors for the different scans? Do you mean just slightly off colors because it wasn't scanned all at once? Obviously an entire scan is going to be the best, but if all we can get is a collection of partial scans, it is at least better than nothing. You could always post up what you can and we can see about post-processing.
Also -- make sure that as you scan the different sections that there is a little bit of overlap. What I mean is - let's say the poster is folded into fourths. You could scan each quadrant exactly, but the best thing to do would be to scan, let's say, the upper left quadrant plus a little bit of the other quadrants around the perimeter of it. This will make connecting the pieces much easier / better.