No, Curse will not be able to prevent you from installing any mod you choose.
Will Curse Launcher be REQUIRED to use Forge? NO, you will still be able to download and install Forge exactly how you can now. The major end user visible change. Is that this forum, will be moved to a Curse controlled forum. And our download list (this) will move to CurseForge somewhere. The same exact files/setup. Just a different, more robust host.