My impression is that caliente would be used to describe a concrete thing, whereas caluroso is more abstract. Pavement or a coffee mug would be caliente. The weather or ambient environment is caluroso.
I dunno how common it would be, but it's certainly a valid use of the -isimo morpheme, and I see no reason why it shouldn't be accepted. If that was rejected, I hope you reported it. One detail -- you need an accent over the first i. :-D
The voice you hear is random. We don't all hear the same voice for a particular sentence. Judging by the large number of complaints in various parts of the forum, you aren't the only one who finds some of the new voices annoying.