In general, 4.7 is a "development" release. 4.6 is a "long term support (LTS) release". Most people would recommend 4.6 (or even 4.4!) for very stable websites, and only using 4.7 if you need the latest features or are developing for the future.
As an extension contributor myself, there's a reasonable functionality jump between 4.6 and 4.7, so it may be some time until every extension is on there, and some may never get there. It depends on the extension and the developers who maintain them.
Unfortunately, there is no way to "downgrade". Your best options are to:
1) Manually download and install the extensions, put them in the extensions directory, and then test them. They will probably mostly work, but you will need to do this testing yourself.
2) Contact the developers of the key extensions, asking them for a timeframe of when they will be supporting 4.7
3) Hire a freelancer or agency to update them to 4.7 for you. I would be more than happy to do this - http://civifirst.com, or you could look for a partner agency at http://civicrm.org/partners-contributors.
4) If you do decide to move to 4.6, you can easily export and import your configurations and data. If you find yourself needing to do this, do ask here in a different thread.