It require's a heck of a lot less skill to tap threads into them after they have been drilled than it take's to drill them with a hand held drill motor. I'm willing to bet there aren't 3 people in your intire neighborhood that can drill them free-hand. But no,common floor anchors don't hold well in metal. I would use a wire welder to attach all thread rods to old metal or just move over and install anchors in steps.

The best way might be to drill a hole in the steel with a steel drill that is oversized. Then using a masonry drill the right sized, drill a hole in the concrete beneath the steel. Then using slam-in-anchor bolts, drive them in with a hammer to set them in the masonry and then bolt down your new post. Then using a grinder with a cut off wheel, trim the overly long slam-in-bolts to length.

Are slam-in-anchor bolts brand name like Redhead or a type manufactured by a particular company?

Probably. They are a type, where you drill a well-fitted hole, then after putting on a nut to keep the threads from being ruined, you tap them in with a hammer. As they hit the end of the hole, they expand, locking them in place.

These work much better than TapCon's which in my opinion almost never work. I use a lot of concrete anchors because of where I work, and use a lot of slam-in-anchors.