Using inline comments for point-by-point.
Titans have void shields right? As I understand it even on a ship it's all the same emitters just a different shield which is why you have to choose which one you want up the void shield or the geller. As big as a Titan is and with the powerplant it has I don't see why it couldn't mount a geller field especially if you mounted it externally.
Re: emitters- maybe. Dunno. Bearing in mind the existence of miniaturized, extremely rare void shields (if they weren't rare, they'd be on Baneblades), I am willing to accept the postulate that miniaturized, extremely rare gellar fields might exist. I'd also expect them to be archeotech, or secrets of the AdMech, much like Titan void shields are. You might be able to do it if you had the means (and a cooperative GM), such as a fantastic reputation in the AdMech.
Better yet if you're that worried about it build it a barge and tow that, maybe it's just me but I'm loath to overspecialize one of my own frontline ships in such a way.
Frontline ships? Oh, heavens no. I can imagine no reason to mount this in anything less than a dedicated transport, a grand cruiser, or a multirole Mass Conveyor- they make great planetary invasion ships. That would, of course, require a huge profit factor to support (as expected for someone fielding titans), so there you go.
The Mechanicus isn't going to just give you a Titan to have forever and do whatever you like with, so why throw all your resources at being their damn cabby? I'm just saying more cost effective methods I think.
Yeah. This is why most RTs don't have a Titan. Being their 'cabby' does take a lot of resources, and they (probably) won't let you keep the titan. You might be able to trade for their support somewhere else though- move a Titan somewhere in exchange for access to the Lathe shipyards? Perhaps some Best (or Best++) quality Ryza macrobatteries? If the paycheck is worth it, I'll drive anyone around.
You start towing their God-Machine through the warp though, kiss your good reputation with the AdMech, your access to the Lathe yards, and most future Mechanicus contracts goodbye. Just my expectation.