Is there harm in using my mini-splits in conjunction with my hot-water baseboard heat?
Running them at the same time won't cause breakage

As Ed Beal indicates in his comments, running both systems simultaneously with the mini-split set for heating won't cause trouble -- in fact, your mini-splits have support in their controls for running an auxiliary heating system while the heat pump runs if it can no longer keep up with heating demand.

However, you'll need to get clever with controls here

Normally, what I would suggest for your type of situation (no significant concern for running heat during power outages) would be to have the mini-splits control the baseboards using their auxiliary heat control facilities.  However, due to the lack of a second floor head outside the little-used guest room, this approach won't work on the second floor, for sure, and also seems not quite compatible with your control goals in general, given that your economic crossover point is rather high.

As a result, you'll need to do some dancing with your control setup to get the right mix of baseboard and mini-split heat for a given situation -- most low-voltage thermostats should be able to handle controlling the hydronic zones on your behalf, although you may need to route C wires to them, while the Flair device you linked in the comments can handle bending the mini-splits to your will.