Would I be able to use break loop instead of the comparison, too?
Yes, you can use "break" - add a sub-event in the loop, in this sub-event check if loopindex>1000, then break loop. But it's easier just to add this condition to the While loop itself.
Also, since I'd like to have it happen on start of layout, cant this also be done in a single pass?
You mean add two cells in one loop? Yes, it's possible: