The other hashtags work too. You can see that if you go to the list of tags on the right hand side of the main steemit page (steemit.com). Click on Gardening, for example, and under Trending you can see posts that had their first tag in some other group. But they are still showing up under Gardening because that was one of their tags. So having too many of your tags be for giant groups, your post might get lost. It's helpful to choose some of those tags on the list to the right. Hope that helps!