Once its settled, the goni should easily win that battle.
Here's a picture I just took of some of the polyps on my colony, it was too close to both some xenia and some palythoa grandis, was easily beating them both back. At the tips of the polyps you can see what I assume must be nematocysts (the stinging cells), they are only present on the largest/most extended polyps.
edit: realized I didnt really answer your question; you'll probably want some more distance between them yes. Goni's can go right on the sand bed, that it how they are often found in the wild so you have plenty of placement options. Make sure its getting good, but varied/indirect flow and you should be fine.