I tend to only keep a few kitties out to breed. If I get some good boxes that I want to breed I put them in my "future breeding folder" and I unpack them when older ones can no longer breed. I try and only have a few pairs out at a time because it does cost money to keep them breeding. When I first started I ended up with over 100 cats because I kept opening ones I wanted to keep. I no longer open every single box. I just stick them in that folder and wait to use them.
Good luck.
And my residential land allows breedables so I don't pay extra for a separate piece of land. I also do not buy vitamins anymore. I keep it to milk and food only.
How do you manage to keep breeding without going broke
