I thought about a trial period, as it would be a good way to test & see if we're a good fit in terms of philosophies & approaches to different things.

But then I wondered about what would happen if I ended up deciding I didn't want to work with this person anymore- how would I compensate them for what their contribution was up until the point that I decided to pull the plug on the relationship? How would they know I wasn't just working them to get some contribution out of them with the intent of dumping them in the first place. I certainly wouldn't want any ill-will at the end of our working together, so I'd want a fair way of parting ways somehow, and I couldn't afford to pay for their work until later when the company makes money.

So that solution doesn't seem do-able anymore.

Certainly better than just jumping in. I think launching a tiny project with someone (something that just takes a few days) is probably a good way to test the waters.