OK asked this in the rules area but not sure if right area so will ask here as well.

Ok I have just read Titannicus and have found this about the Divisio Mandati of the Titan Legions.

Divisio Mandati

The Titan Orders of the Mandati are known as "Executive Orders". They travel in vast temple-spacecraft, responsible for bringing the Pax Imperia to independent human worlds. Each temple ship carries a small but potent number of the immense Emperor Titans, plus priests, regular troops, maintenance adepts and support personnel.

Each Emperor Titan is its own self-contained unit, carrying members of the Adeptus Arbites, the Inquisition and the Adeptus Terra. It is through this combination of persuasion and vast firepower that many worlds are brought into the fold of the Imperium.

The elite of the Mandati's orders are known as "Missionary Orders" - they travel beyond the known areas of space, often accompanying Rogue Trader missions.
Divisio Telepathica

As a highly specialized Divisio, the Telepathica is somewhat smaller than the Mandati and Militaris Divisios. The Telepathica is responsible for the dreaded Psi-Titans. Its orders are the Telepath Orders, each said to operate from a secret forge world near the heart of the Imperium.

So I have 2 questions , are there rules for carrting titan Legions on board a Rogue Traders ship and are there rules for the Divisio Mandati.

Thanks

Not in the core rulebook, and not in the core rulebook.

I believe THAT level of power will probably left for the next 40k rpg core book, even then...

That's kinda more along the lines of 40k Apocalypse or older games lines.