The mild steels I have come across and cut have been:
CRS (Cold Rolled Steel) which most sheet metal from 22 GA (0.030") to 10 GA (0.135") usually is sold unless you specify HRS. Very easy to cut.
HRP&O (Hot Rolled Pickled and Oiled) usually comes in thicknesses from 0.187" (3/16) to 0.375" (3/8). I have a lot of this type of material mainly on 0.187" and 0.250" thicknesses and it works great. Usually the material is free of rust and scale.
HRS (Hot Rolled Steel)mostly common for thick plate but you can also get it in smaller gauges. This works out the best for cutting really thick materials from 0.312" to 1.125". Of course, the better the surface quality and grade (A36, A572, etc.) the easier it is to cut it.
This of course will depend on your customer material requirements.
Hope this helps