Rule of Three. HOWEVER:
Do they still have -1 to hit?
No.
What benefits do flyers have now?
More penalties than benefits. They can more easily see over ruins, but can be more easily seen. Also, unless you choose to use Hover mode, they MUST start in Strategic Reserves.
Also can flyers hold objectives?
Most aircraft have OC 0, so no.
Also can titanic units like the stompa and morkanaut shoot while in combat, or fall back and shoot?
Monsters and Vehicles may shoot while in combat; see 'Big Guns Never Tire'. Generally, no units can Fall Back and still shoot, though various characters, bespoke rules, stratagems, or detachment rules may allow it.
Can I use the Dakka Dakka Dakka stratagem and bigger shells for bigger gitz stratagem on the stompa and mork/gork units? It doesnt exclude titanic.?
As long as the target fits the condition of the stratagem, you're fine. Titanic units are only excluded if the stratagem specifcally says it does (see: 'Fire Overwatch').