Larry Niven also seems to be quite obsessed with inter-species sex (though, in the case of Ringworld, I think it's technically intra-species sex).
Don't think so; "rishathra" was defined as "sex outside of one's species, but within the hominids."
Well, the Ringworld species are all descendants of the Pak (who, for the sake of simplicity can be considered human), so they're all within the clade "humans". Whether you consider them to be different species, or different races* within a species is a matter of semantics, and depends on which definition of "species" you use.
One of the classic definitions (the Biological Species Concept) involves reproductive isolation, with any two populations that are capable of interbreeding being considered the same species. If the Machine People can breed with the City Builders, and the City Builders can breed with River People, and River People can breed with Ghouls, then all of these groups are members of the same species.
I can't remember if interfertility was specifically addressed, but either way, depending on the definition you use, you could make a case either way; whereas with kryptonians and humans (or klingons and humans) there's no way to consider them in any way related.
Oh, and "hominids" is... well, not wrong, exactly, but misleading. The hominid group includes chimps and gorillas, but not baboons and gibbons, and is a very arbitrary line to draw. All the Ringworld populations are definitely hominoids (members of genus
Homo) and, for that matter, if we do define them as separate species, they're all members of the family
Homo sapiens, so a better definition would be "sex outside of one's species, but within the humans", though that might be somewhat confusing.
* Biologically "race" encompasses far larger variations than are seen within populations of humans outside of science fiction. There is only one race of humans