Familiar plants?

I thought that, barring specific Virtues such as Spirit Familiar, only animals could become (Hermetic) familiars. However, I noticed that the Criamon on the Path of Walking Backwards are described as occasionally having magical plants as familiars (p.72, last sentence of Path Symbolism insert). Is it something any magus could do? Did I miss anything?

Noting that you already knew about Summoning/Spirit Familiar/Theurgic Spirit Familiar, to my knowledge spotting that bit in House Criamon means you haven't missed anything. Also, you could still be right with your original assumption. It is a Mystery Cult after all. So maybe there is a Virtue that can be initiated (surely minor) that allows for plant familiars.

I have seen and made house rule allowing for plants, elementals, and some other things to be familiars, sometimes with a Virtue and sometimes not.

Realms of Power: Magic lists on page 57 a mandrake as a possible familiar bound by using Herbam and it's stats are in the same book on page 130. It says that it may require a story to investigate the process to turn a mandrake into a familiar or that the storyguide could just allow it.

Yup, like with the Criamon the books essentially say, "It's possible, but we're not going to tell you how and it may require more than vanilla Hermetic magic."

Legends of Hermes also has a whole chapter related to magical plants.