1st question: yes, although you can still keep existing partitions if you want. Normally with windows, it hogs the whole disk - not usually set up with spare partitions. I would recommend getting a spare HD and putting fedora on that.
When you set up the partitions it is possible to put the whole thing in root, but I would advise splitting it up a bit. It will probably force you to have at least a /boot partition of at least 500Mb but bigger is better 1 Gb say. Sounds mad, but it uses this space to upgrade to the next release - better to have to big than too small.
Other candidates for partitions are your home directories, /opt for additional software, /usr for std software, a partition to test another version of Linux (Ubuntoo say), a swap partition if you don't have much ram, the list goes on.
Also recommend LVM (Logical Volume Management) which are resizeable partitions - very convenient sometimes.
So, overall have a good think about how you are going to set it up.
2nd question: No. You can download all kinds of different versions.