So after a little discussion with Psykers, I've decided to adjust the seeds just little bit. For seeds that clash with one another, for example, Duchess Bastille and Lady Haliel, instead of the duchess' seed completely canceling out Lady Haliel's, what's going to happen is there will be a battle of wills over which effect gets played. This battle of wills can be played out like a fight, but if I need to toss a coin to ensure one prevails, I can do that.

This makes interactions way more interesting, I think. ;) Of course, this is only in regards to seeds, not to magic.