Can these be attached to a 6x6 post with a sleeve?

Jesse, Yes, you’ll just need to pick up separate wood screws, since the ones included are designed for aluminum posts. Those factory screws are lighter-duty, and pressure-treated wood requires stronger, corrosion-resistant fasteners for a secure hold. I’d recommend exterior-rated deck or structural screws long enough to go through the bracket, sleeve, and into the 6×6 post. That will make sure the railing is solid and holds up over time. -Your Deckorators Team