You can of course use an extension tube as a lens hood.
Your aperture value won't be "correct" when you reverse a retrofocus lens like a wideangle. It'll be smaller, effectively, than the lens says. That only matters if you're trying to calculate depth of field ot diffraction limits - but you're in "try it and see" territory, anyway.
Some standard zooms can be surprisingly good, reversed, even humble ones.
If you're using a lens on tubes, you may well find you have to add "flocking" to reduce internal reflections inside the tubes or from any adapter. They can be troublesome, giving reduced contrast. Black art paper is OK, but something like Protostar is better. Cheap tubes are the worst for that.
You usually get better peformance especially at the edges, if you put a short prime reversed on the front of a long prime, with both focused at infinity, but predictions can go wrong with specific lenses.
For single reversed lenses, such as 20 - 50mms, best aperture is usually about f/6 for a f/2.8 lens. smaller will give more DOF but everything will go soft from diffraction. You get much sharper images if you focus-stack at the best aperture. Just about impossible for live subjects of course.
If it's your first go at high magnifications - use flash to avoid motion blur, and diffuse like crazy - eg a sheet of paper wrapped around the subject and lit as evenly as possible, works.
