What tag should be used anywhere or what tag is actually available here to be used? Seems like it just falls under monster girl most of the time due to a lack of options.
I think outside of adult games they're scalies, but since we don't have that I think either monster girl, fantasy, or furry would fit because of what those terms imply in a larger scale
I non-human protagonist if it's the MC, or monster girl if it's one of the girls, I don't think you need something specific for a reptile if that is what you are asking, as then I think you would need one for every type.
Generally, unless the creature's obviously monstrous in the imagery it's meant to evoke, I'd go furry. It's basically a furry offshoot anyway, called scalie. Monster tags are reserved, in my mind, for when the character is more bestial than human, in either mind or body. Or both. Think the characters from Snoot Game versus the more mindless iterations of The Lizard from the Spider-Man comics and TV shows.