Well, maybe.
Right now Photos doesn't even have an "edit in..." function, and that won't change I expect before release.
There is no "media" sidebar item in Finder windows in 10.10.3 beta right now, so even getting at the Photos photos was a PITA; I would hope that would change with the final 10.10.3 release...I can't imagine you couldn't access Photos at all.
You probably have some extensions right now; go to System Prefs and check 'em out. Pixelmator installed one called "Repair Tool." If you drop a photo into an email in Mail it lets you remove stuff...sort of an interesting choice of tools....
On the iOS side I don't have Pixelmator but I do have many many other photo editors. You can access some from within iOS Photos, like Camera+ or Afterlight. You edit away, and it saves the edited photo back into Photos. I would hope Photos in Mac OS could eventually be able to do at least that.
But as usual with Apple, we're left guessing. I wouldn't recommend plopping any money down on anything that is reliant on Photos and/or iCloud Photo Library until some time has gone by after release. Pixelmator can work just fine by opening photos from the Finder and saving them back there; Photos doesn't really add anything to that. So just ignore Photos and use Pixelmator and you should be fine.