No, you don't have to pay for future updates.
That's difficult to say, as there's no default "actor portrait" interface in the RPG Maker's core code that we plugin devs could use, so every plugin devs make their own implementation of actor portraits. You can tell my plugin not to show portraits, so the other plugin can render hero animations; however, when having multiple plugins to customize the UI elements always has a risk of incompatibilities.
You can tell me the name of the other plugin and where to get it; and if you notice compatibility issues and I can try to fix them, and if I fail I can also offer a refund.