At present there are three kinds of hybrid taxa available to choose from when creating a new iNaturalist taxon: GenusHybrid, Hybrid, and InfraHybrid. I think some questions may need to be addressed before settling on a general rule for hybrid taxa in iNaturalist.
GenusHybrid: intended for hybrids between different genera, parented directly to the next higher node containing both genera. (Lots of examples in Poaceae tribe Triticeae.) A named hybrid would by definition be between two taxa of the same rank (genus). For example Genushybrid ×Elyhordeum.
Questions: when creating a GenusHybrid taxon using a hybrid formula instead, would we want to allow such formulas to specify subdivisions of different genera (like subgenera or sections) for one or both parents? Seems like that could get pretty messy, especially as infrageneric nodes are revised over time. If we do allow such formulae, should they be restricted to the lowest-level infrageneric rank(s) recognized within each genus? Or are we ok with GenusA × GenusB section C, even when GenusA contains subgenus and/or section nodes?
Hybrid formulae between different subdivisions of the same genus may be a different matter, but only in that the iNaturalist rank GenusHybrid may not have been intended for them. As hybrid formulae, however, they are easy enough to accommodate if desirable. Personally, though, I would prefer to keep things simple here and restrict GenusHybrid taxa to hybrids between taxa of rank Genus.
Hybrid: similar issues and questions as above. Intended for hybrids between different species, parented directly to the next higher node containing both species. A named hybrid would by definition be between two taxa of the same rank (species).
When using a formula instead, I think infraspecific taxa could be included as long as they belonged to different species. But again, should they be restricted to the lowest-level infraspecific rank(s) recognized within each species? Or are we ok with speciesA × speciesB subspecies C, even when speciesA contains subspecies or other infraspecific nodes?
InfraHybrid: not as certain about the intent of this rank, but if analogous to the others, it would be for hybrids between different subspecies (varieties, etc.) of the same species, parented directly to that species. Hybrid formulae between subspecies belonging to different species would fall under the rules for the iNaturalist rank Hybrid instead.
Because of the nomenclatural equivalency of the different infraspecific ranks in plants, I think we have no choice but to allow InfraHybrid formulae between taxa of different ranks, as long as they are both infraspecific ranks within the same species.
I’ll note that the ICNafp Article H5.2 states
If the postulated or known parent taxa are at unequal ranks, the appropriate rank of the nothotaxon is the lowest of these ranks.
If we are only dealing with hybrid formulae, I don’t think this impacts any of the above, but if hybrid names (nothotaxa) are involved it could become an issue.