Will these fit my factory wheels or only aftermarket?

Acorn lug nuts work with any wheel that has a standard conical seat — factory or aftermarket. The extended length gives you extra clearance if you run wider offsets or lowered height, so they'll work in both situations.

Do I need any special tools to install these?

Standard lug wrench or impact socket in the size for M12 hardware is all you need. Installation is the same as removing and reinstalling your OEM lug nuts.

Can I mix these with my existing lug nuts, or do I need to replace all 20?

You should replace the full set on all wheels to maintain even clamping load and appearance. Mixing materials and styles across the same vehicle can cause uneven pressure on wheel seats.