My Spanish is functional but I'm not a native speaker. I think it would be serpientecita or serpientita. "Serpiencito"seems like it leaves off too many letters from the root word. Also, the noun is feminine, and I'm not sure if when you use a feminine noun pejoratively on a male, that you'd change the gender to match. I *think* it'd stay feminine. (LOL Patriarchy)
...Random google thing also tells me that many Mexican Spanish speakers avoid this word because it sounds too much like serviente. No idea if that's true, though they do sound similar. Culebra is another word, no idea how popular in Mexico. There are also other "snake" words. Just like any language, Spanish is rife with double entendres, with all sorts of variations through the many countries that speak Spanish. Strongly suggest you find a native speaker or two to run through all your Spanish at some point, especially when you're talking about phallic things like snakes.