Concrete – Knowing where to drill holes in steel plate

Given that your stringers are going to be about 8" wide, you'll be fine with some pretty thin stuff. Heavy Jeep frames from the 1970s were only about 3/16" thick, and only about 4" wide. I've seen truck frames around 1/4" by 8", but they were larger single-axle agricultural trucks. Really large single-axle trucks only have about 1/2"-thick steel frames.

Come to think of it, I've walked stairs made with no heavier than 3/16" stringers in penitentiaries. They felt nearly like STONE.

If you're concerned about racking, remember that every tread will act as an anti-rack brace. Until your treads go on, the stringers will feel pretty flimsy if you push them sideways. AFTER the treads go on, they'll be train-wreck rigid.

Steel shims if shimming steel and concrete - either with holes that you bolt through, or tack weld them in place. To the extent that you can use "fewer, thicker" shims rather than more thinner ones, do that. Paint, galvanizing, or "cold galvanizing" (zinc-loaded paint) may be advisable to limit/reduce rusting over time.

The other solution, though it's not usually referred to as "shimming" would be to use mortar/grout to correct the concrete beam surface for good bearing of the steel in correct orientation.