I've never grown the Russian Giant but have grown others like them. You'll definitely want to bag the flower heads once the seeds start forming otherwise the squirrels will steal them. Panty hose is a good for doing that. The picture is what a sunflower looks like a few weeks before harvesting. The outer seeds ripen sooner than the inner seeds. I usually cut the flower off when about half the seeds are ripe then hang it upside down for a few weeks before harvesting to let it dry out a bit. As to crossing, I don't know but it's possible if they are the same species.
When you go to harvest the seeds you'll want to wear glove because your fingers will get raw from scraping them off.
