I'm not sure what you mean by Yosemite defaults?
Presumably you mean like with a new user. If you mean settings saved in your Apple ID, you can sign out of iCloud in system preferences. It sounds like you want to start fresh. If so, just set up a new user in system preferences > Users/Groups and manually migrate whatever data from your original user. You'll need to select administrator access when prompted, and may need to reset permissions.
Best option I'd think is to use Migration Assistant with a new user. Here's an Apple link with explanation. This will set permissions for you (just use spotlight to find, or look in /Applications/Utilities.
This may not be relevant to you if you're using an Apple installed SSD. If you're using a DIY SSD, Trim support is tricky in Yosemite because of recent modifications to kext security management that affects Trim support. More here about Trim Enabler for Mac.