Honestly IMO I've always seen the "ally" flags as kind of... cop-out ish? Like even though it's probably not the intent it's always odd to me when i see one, and gives off the impression of needing to specify "I'm not one of them though!" for a safety net against xphobia that the rest of us don't get to have. My partner is cis but he sports a trans flag to support us and to show he's not afraid to do so with his whole chest, and seeing that rules way harder than an ally flag.
Just use the regular pride flags, so long as it's with genuine support it's not weird!