Tank Regiments contain just 1 kind of tank as stated above, its a quote from SLSB that I think is repeated in FM:SLDF.
Infantry & Armor are not mixed in line regiments.
IMO, the closest you might get is the Independent Mech Regiments where you might have a Company a Infantry attached & also have a Company of Rippers.
I think attempting to "mechanize/mobilize" an Infantry Regiment is best shown by either flat out adding a Battalion of APCs to it, or going the BattleForce/4th SW chart route where "Mechanized" infantry regiments actually consisted of 3 different battalions, 1 Leg Infantry, 1 Motorized Infantry, 1 Light Tanks. Only in this case I'd make it Light IFVs.