Forms are not available names under the ICZN, so I would think they should not be added to animal taxonomy on iNat. And while not explicitly stated in the curator guide, I get the impression that curators should only be adding clades. I think the rank may exist in our system for plants/fungi which are governed by different taxonomic standards.
You might be interested in some similar comments here:
https://forum.inaturalist.org/t/what-happened-to-the-taxonomic-rank-variety/31058
But all IDs are based on a taxonomic framework. Mixing frameworks seems like a bad idea to me.