Seems to me if devs want to increase cash flow to some extent, allowing more pet slots per lot would lead to more shop purchases. So many pets, and no ability to use them. (washing my mouth out with soap).
They are very resource heavy to have in houses based on what has been said before. I would like though if we could separate pets from vendors though so we can have at least a few more options. With NPC's for inky I can see a need for this to get increased even if by 1.
I have 140 NPC slots in my dungeon. That means I can place ALL of my pets down there, and have them give quests. LOL!! Walk up to my giant squirrel pet and he asks you to bring him more nuts...which my automaton pet has taken for examination a few rooms down.