Okay, if you took the serpentine form and then took the legs evolution I would say you lose the immuniy to trip. However due the the extra set of legs you would benefit from stability and receive a bonus to CMD vs trip. Also, the RAW I believe says pounce is for quadrapeds only, but not quadraped base forms, so I would say you can take pounce. Finally, you're still serpentine, so you can take constrict.
But after all that why not start with the quadraped? Although I get a cool visual from what you describe as this lithe snake creature with stubby legs running super fast to give out big hugs to bad people.