Grav-shields are not cumulative - just tried it. Otherwise, I agree 2 or 3 fusion defences are more than enough. It's not worth building any defences weaker than plasma.
You CAN get attacked if you have 1 base and it CAN happen in the first month of the game. I remeber I once captured a Sectoid Commander in the first month that way!
P.S. Are Battleships the only ships that can attack your base? In X-com 2 I've been attacked by both Dreadnoughts and Battleship (in X-com 2 the battleship is equivalent to the terror ship from x-com 1).