I reviewed the DB code this morning and here's how it works:
When you make a new parent plant it will find all the matching plants and make them children IF each child plant does not already have a parent plant.
So since all Plectranthus scutellarioides already have a parent plant, then by creating a new parent plant in Plectranthus, then only those plants in that genus who are NOT child plants, will be added as child plants to the new entry.
In other words, there should not be any conflict here and you are safe to mark Plectranthus as a parent plant and it won't affect the coleuses.
