Yes. You can add drives as you go (both full or empty). You can also install flexraid later (after you are up and running )
Build a server and get it working, get some storage. Once you have 4 or more hard drives, add flexraid. It installs into Windows or WHS pretty easily.
Full drives will take a while to get parity going, (how long it takes to read all the data) but aside from that there is no downfall. I like to add empty drives because it's easier, ( and faster)
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."