The most common type of company found in regiments raised on Armageddon are Mechanised Infantry Companies. These are similar in most respects to normal Infantry Companies, except that all the units that would normally have to move around on foot are provided with Chimera armoured transport vehicles. Mechanised Infantry Companies are normally quite rare in the Imperial Guard, as it is difficult for most planetary governors to obtain and maintain the vehicles needed for such a formation. However, Armageddon is one of the chief manufacturing planets for Chimeras, producing several hundred every day for use across the Imperium. Because of this, infantry units raised on Armageddon are routinely equipped with Chimeras, unless the tactical or strategic situation makes their use impractical.
Using a Steel Legion Mechanised Infantry Company in Warhammer 40,000 is very straightforward. It is simply picked from the standard Imperial Guard list in Codex: Imperial Guard.
Deathworld Veteran units from Codex: Catachan may not be used in a Steel Legion Mechanised Infantry Company.
All units in a Steel Legion Mechanised Infantry Company must either be vehicle units, or have a Chimera transport vehicle. Units that don't normally have the option of taking a Chimera transport vehicle, such as heavy weapons teams, must do so at +70 pts, plus the cost of any upgrades taken.
Rough Riders may not be given a Chimera, but may still be included in the army as it's assumed that they are fast enough to keep up with the rest of the units.