what do you guys think are realistic feeding gains? is it possible to do 100lbs, 200lbs? even with a lower initial weight?

It's physically impossible to be 1000 lbs for a human, let alone 2000.
Imo girls that start skinny usually stop at around 400 lbs and never go beyond that. That's why most 500+ lbs SSBBWs were never skinny and we're always fat at a young age.

Realistically, if you get a girl who's around 120 lbs you can at most triple her weight.