You shouldn't use degrees for Kelvin, it's an absolute unit, the degrees are needed for the relative units like Celsius.

Anyway, the French system isn't what people mean by "metric" in this context, they mean the SI system of units, and so in practice it's not so much that it wouldn't matter which you choose as that you don't have any option except SI.

If you wanted an independent system of units you'd need to do a lot of expensive metrication, and in practice Americans are too cheap for that, so the US "customary" units are just aliases for so-and-so-much amount of some SI unit, they aren't actually independent at all.

The reason people focus on metric is that for everyday people that's the part which jumps out as more intuitive. All these nice powers of 10, very tidy.

It's useful in STEM because SI units are often straightforwardly defined in terms of one another, it substantially reduces the amount of arbitrary constants you need in your equations, making them a lot easier to work with.

However the abbreviation is STEM. For S and especiually M you can do in different units.

For T&E it really matters see NASA's Mars Climate Orbiter and the need for heroics in the Gimli glider.

You need to keep to the same unit.