80F is too hot for horses. You need to get the temp below 75F IME.
24g would be to hard to keep dwarfs in.
There are mods to get the temp down, what model of nano do you have JBL, Aquapod? Some of the mods are very easy and will work great, others are more costly.
I would go with a pair of Kuda, they are hardy and available in the CB aquaculture environment. An Auquacultured horse differs from a TR or CB horse and is easier to have good succsess with.
I do think Caps would do well in a tank that size but the mod to keep the temp at 65F or below, would be expensive. Also caps aren't tht easy to come by. Brevies or whitei would do good, but same temp needs.