Ya you basically want to be directing your tungsten toward the base metal prolly 3/4 of the time, and then wash up briefly to melt the 16 guage tube and dot your filler in there. Not really somethin most would start out with, but if you've done it with mig (or stick) you should prolly get what your lookin for after a few practice runs on scrap.
For the cup size- the size of the electrode is only one of the factors dictating cup size (actually its prolly the last). Type of material (I assume your doin stainless for the exhuast?) is the biggest factor, because that will dictate gas flow. For stainless you really need a gas lense for the uniform gas coverage-the added, second benefit of a gas lense is it allows you to stick out your tungsten WELL beyond what you would normally be able to with a standard cup.
BUT, in a pinch, yes, you can crank up the gas flow and stick the tungsten out more than you could with a regular cup, although again I'd recommened using a gas lense, especially if yuo have to use a small cup to fit in a tight spot.
You also might want to consider a non-standard torch with a smaller backing cap. This would allow you to use tungsten around 3" (some, like the pencil-type torchs, use even smaller lengths tungsten) and might help you get in tight spaces better.
edit-welding differant guage material with tig, having a foot pedal would be exponentially helpful
