The Royal Battlemechs were upgraded Star League designs used by the old Royal Divisions of the SLDF. An example would be the TDR-5Sb Thunderbolt, which had improved weapons and protection compared to the regular TDR-5S. They appeared in the Operation Klondike book first IIRC. They're statted in Operation Klondike Record Sheets and Record Sheets 3075 The Age of War
To be honest I don't think they'd make a huge difference. The ComGuards would have had a few but the bulk of their forces were still standard Star League equipment, along with the 'Clanbuster' refits. The Com Guard won through tenacity and cunning, the Clans lost through bone-headedness and arrogance. A scattering of Royals won't change that. Might be fun to add a few as flavour of course, or just because you like the mech.