Purrely is a archetype all about building up to big xyz by feeding your pets(monsters). most cards in the archetype are good enough to be played at 3 with some exceptions. The Happy xyz line are alright but you arent likely to go into them much until your forced to go second, then they can be good tools. The Field spell and Trap are good but searchable so 1-2 of each is better than 3.
Card Tech Explanations:
Generation force/Xyz gift: Pot of greed(kinda). Your go to play should be making a plump then while your feeding it keep spitting out Purrely till you can make princessprite(as she is effectively a draw 1). then once both are set up and you used her effect rip the last mat off her and one mat off plump and enjoy your draws.
Shadows Light: Free purrely summon from deck without the discard cost so its good card economy, simple enough(with added bonus of being able to banish it later for an extra normal if you ever feel the need.
Dark World Monsters: They are discard fodder, they let you go deep into your monsters without slowly killing your hand(with these you can easily get 2 noirs up turn 1).
Overlay Network: This ones a more experimental choice, as plump can turn ANY spell/trap in the grave into a material so overlay network can let you reuse and spell/trap that you want, incredibly flexible use case in purrely.