Personally, I've mixed and matched the Little Twin Stars/Kiki & Lala and My Melody furniture with not only the mermaid furniture as you mentioned, but the princess, sweets, and sloppy set. You can also customize the kiddie set to pastel and the ranch set to white. I've seen other people use parts of the Weeding Day set, regal, alpine and minimalist sets to tie in a cohesive theme. You can also customize the bears to cute options and I really like the mama polar bear in particular.
Some furniture I can think of (don't belong to a set) that can be used too:
- pastel low table
- white festive tree
- afternoon-tea set
- rice cake
- Poogie
- carriage
- beauty salon cart
- bathtub
- box corner sofa (can be customized to white)
- chaise lounge (also can be customized to white)
- chocolate fountain
- ebony piano
- marshmallow chair
- soft-serve lamp
- trunk (customized with an in-game clothing item like a candy gingham shirt)
- cushion
- aroma pot