The categories, tropical, sub-tropical, and hardy are simply too vague to use if you really want to do well with your trees. You need to do research on each species you grow. Some tropicals are heat lovers (thermophiles) for example Cacao - chocolate, temperatures below 45 F can kill the tree outright. It goes dormant if it cools below 68 F or somewhere around that point. Others are more tolerant of cool, will stop growing below 55, but are not damaged by short term exposure to cooler temperatures as long as they are above freezing. It is species dependent. So how to get to a useful answer to your question?
Like BrianBay9 said, look at the places you have available to winter your trees, and then sort your trees by what they will be exposed to.
Serissa is a species I have yet to figure out, I have yet to keep one alive for more than 3 years. I have one entering its second winter with me, I am going to keep it with the orchids, as a tropical this winter. I think Anthony touched on the key, Serrisa come from a pretty wide geographic area, and there are likely geographic differences in their ability to tolerate cold. Some cultivars may be more cold tolerant than others because of their origins. Only way to know is to test them. Start with several inexpensive young plants of each cultivar, and put some in each of your wintering areas, see which survive.
Similarly, I know Cornus florida, White Flowering Dogwood, if the seed source is from Florida or southern Georgia, it will not survive a Chicago winter, if the seed source is from New York State, or Pennsylvania, it will survive Chicago winters just fine. Geographic ecotypes, genetically different populations, though still similar enough they do not get recognition as sub-species or separate species.
One thing to consider, I don't know where the line on the map is exactly, but I believe in zone 7b and warmer, it is rare the ground freezes to any great depth. Many zone 8 trees will tolerate the trunk and branches receiving a freeze, but will expire if the roots freeze. So even if the "books" say it is hardy to 15 F, that may be true for the trunk and branches, but it might not be true if the roots were to freeze. So in general, your sub-tropicals, 7b and warmer, should be brought inside before it gets cold enough to freeze the soil in the pots, and situated where the pots won't freeze.
