ToxicStarShipping - Koga x Shauntal
Toxic --> poison --> Koga specializes in Poison-types. Shauntal's Japanese name is taken from the Japanese star anise, hence the star part. And the plant happens to be highly toxic.
VillaShipping - Steven Stone x Caitlin
Both have owned a villa at some point (well, in Steven's case, it's more of a "probably owned").
CalderaShipping - Cress x Flannery
A caldera is a crater formed from the collapse of land after a volcanic eruption. Flannery lives near Mt. Chimney, a volcano. Over time, many calderas fill up with water. Cress specializes in Water-types.