I will only need my last certification when I need to attend ATP. Will i be able to do this and then fly as an instructor for ATP?
From your post I am not really sure which certification you are asking about, but ATP generally only hires CFIs that went through their entire Airline Career Pilot Program.
The Regional that hires you will train you and get you your ATP while you get your Type Rating. As Chris said ATP hires it’s graduates.
Where do you think I could go to gain more hours as a flight instructor?
I have no idea where you are, what flight schools are near you or if they’re hiring? If that’s the route you want to take YOU’RE going to have to do some legwork. Get out there, visit your local airports and see who’ll be hiring in 2yrs when you’re ready (chances are they’ll tell you come back in 2 yrs).
Not trying to sell you but when we say ATP’s guaranteed instructor position is a big deal we’re not exaggerating. There are hundreds of licensed pilots out there who can’t get an interview or get hired because they can’t build the time and without it you’re dead in the water.
Really not trying to be a jerk, but why not ask Cindy B. Stephens how you can build the 1500hrs? Seriously.