They are based in traditional mexican hot chocolate, their hotness and spicyness are why they are a fire type, as they are so hot they constantly produce steam, their spicyness is a reference to how originally chocolate was made with chilli, this is also why there are some little chillis included in the design
their shiny colors are based on "Arroz con Leche" a traditional sweet rice pudding often served hot and also spiced with cinammon, just like chocolate
SINISTEA is based on a Mayan spouted cup, the spout is used pretty much like a straw to drink the chocolate
POLTEAGEIST is based on the Rio Azul chocolate vessel, a pot that was used to store chocolate
The wooden club POLTEAGEIST is holding is a "Molinillo", is a wooden tool used during the preparation of Chocolate to mix the solid chocolate with the milk or water in the pot, you need to give one to SINISTEA for it to evolve into POLTEAGEIST