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You might consider using a concrete saw to create a straight edge on the boundary pieces, and if you're really fancy, a chamfer (angled facet) at the top of that straight edge. I would also use a brass tile border to create a little detail between the slate and the hardwood. Home Depot carries these - they're simply a long piece of L-shaped brass, where the bottom of the "L" is webbed to allow thinset mortar to fit inside. Cost is about $15 for a 6' section. I think they come in sections up to 12' or so. We recently replaced the carpet in my son's room with ceramic tile because of his allergies, and the brass border really gives it a nice touch. Good luck with your project!