I mostly use lilies, cosmos, mums, and hyacinths. Warm colours (red through yellow) go up into the upper tiers where my hiking paths are (no idea why, I just like these colours). Cooler colours (blue and purple) are around the music area and overlook over the secret beach. For villager houses, I use flowers that match their aesthetic.
In terms of mixing flowers, I tend to sort my flowers by colour rather than type. So I put a ton of orange flowers near each other in a patch, but it doesn’t matter the species