May's chassis definitely didn't exist prior to her meeting Dale. She may not have had a gender before that.
Her anime maid form--which her chassis is based on, down to the color--was algorithmicly generated based on what the records said Dale would like.
Despite the custom color, shape, and hair, the chassis is clearly meant to be cheap. It has visible joints in places Momo doesn't and hair material that is not as robust as Momo's, where Momo has color changing hair that is clearly easy to style and realistic enough outer skin to be mistaken for natural human twice, at least.
When QC started, Jeph tossed in Pintsize as the obligatory crazy mascot character. He was definitely Marten's property at that point. More over, ALL anthroPCs were fairly sociopathic. Including Momo.
As Momo and Winslow developed diverging personalities things got a bit odd. We meet Marigold because PS gets hurt, and Momo is concerned for her "friend." By the time Momo gets her new chassis, she doesn't even like PS and barely respects him as a person.
The shift from property to people occurred sometime around the Singularity (which, BTW, Jeph has exactly right except that everyone knows it happened. When {if} a technological singularity happens, we probably won't notice anything special). Between then and strip 1900, there was an amendment to the Constitution (United States) granting AI equal rights. The logically consistent way to view the property/people conflict is to view them as property until about strip 1780. But understand that an effort, probably a decade or two in the making, had been pushing for citizenship rights. The logically consistent way to view the change in Momo's personality is to realize that Jeph is a seat of the pants writer, and Momo changed as he found more uses for her. This is exactly analogous to the fact that semi-punk girl, smokes in the bar bathroom, Henners is fundamentally inconsistent with Hanners we got in the end. It's clear that they are the same character. There's hints that Hanners has been awake far too long when we first meet her. She was definitely supposed to be OCD. But she developed crippling hypochondria and extreme social awkwardness over time. Jeph has flatout stated that he was still working out her character.
Momo was a sight gag and joke about hentai. She developed into a fully realized character. Unlike Hanners, Jeph hasn't added a bridging explanation (Hanners was really high on brain meds) for the change in Momo's behavior, so it best to assume that those scenes didn't happen, or that those not consistent with her current personality involved an anthroPC that had the same chassis (and may have also been named Momo). Beyond that, try not to think to hard on it.