Originally Posted by hypo

Wait, I just got to 50 FG, do you mean I should get new slots instead of trying the gacha?

Well, if you get to the point where you have to scrap girls to free up some slots, then you're probably better off getting some additional slots instead of using those gems on gachas to get girls that you have to scrap again anyway because you don't have room from them.

At a rate of 5 slots for 1 FG, the cost isn't too bad. I usually buy them at increments of 25 or 50 slots at a time, to keep the cost in FG managable. That usually lasts me for 1-2 month, and by that time I'll have gathered plenty more FGs again.
I also never stock up on manyu's for longer than a day: at least once a day, I use every manyu I have in my storage, even if that means upgrading girls I never intend to use.

I know some people also sold their 2* and 3* girls, to save even more space. I never felt the need to do that, and managed to get by most of the time with some 30 free slots to spare (which I find a convenient amount for when I want to clear out gacha seeds)