The interaction of character elements makes the most sense when interacting elements within a single quad. Describing interactions between character elements from different quads is fine, but there won't seem to be a lot of identifiable ground for conflict other than general agreement or disagreement.
So, don't feel inhibited from interacting any elements. It creates interesting storytelling.
You can show personal information about characters other than the MC and IC, but beware. When you begin to create personal experiences with OS Characters, your audience will be inclined to see them as MC's. This is fine if your creating substories for these characters. However, if you're not, you run the potential of confusing your audience.
CAVEAT: This seeming limitation is only regarding the Grand Argument Story that you are telling. Digressions and substories are open to all sorts of exploration--personally or impersonally. Just make sure YOU don't get confused so your audience doesn't get confused.