heavy duty workbench leg from metal pipe?
I would put a threaded floor flange on each leg and use that. It also has the added benefit of having a slightly wider base than the cap would and it would have a flat bottom, since it's designed to sit flush against a surface, while the cap is not designed for that.
A threaded cap would work, as well, using the threading as your height adjustment. It may not be completely flat, however, so this might not be ideal (allowing for some of the rocking you're trying to adjust out) and concentrating weight on smaller spots, increasing the likelihood of damaging the floor below.
You could drill a hole in the end cap, tap it appropriately, and run an adjustable furniture foot into each one. This would take a lot more work, however, and would only be of value (IMHO) if you needed more adjustment range than the cap itself could provide. If you do this, you'll have the adjustment range provided by the cap and by the foot, so if you were to set the table outside on a lumpy lawn, you could probably get it leveled even there.
If you use a floor flange, you could lap it smooth using some wet/dry sandpaper on glass until it was smooth enough for you to feel that it won't damage the floor. By doing this, you might not feel it's necessary to provide any protection between the metal and the floor. If you do, be sure to ease the edge - as you're flattening the bottom, you'll also be creating a sharp corner that could scratch the floor or scrape a bare toe.
For any of the metal-on-floor solutions, you could use felt of an appropriately dark color (black to match the metal, or brown to match the floor) as a pad to protect the floor from the wood. I would think that a good double-sided carpet tape would hold the felt on just fine. The cap may need to be smoothed for additional stability and that would have the added benefit of helping the felt to stick better.
I can't envision your third &quot;conical piece of steel&quot; option, so I'm not sure what you're after there, but, all-in-all, it sounds more complex than either of the two above.