unless nerites have suddenly developed a taste for shrimp i cant see why not have 3 of them in my 20g and they do nothing but scrape algae off glass/decor. i think 1 per 5g is best so they have enough natural algae to eat etc but if you were to supplement their diet that would be fine. with shrimp in there too you'd need to add algae wafers or something so they dont all starve i'd think.
. i put some malaysian trumpet snails in with my obi. every time he saw one he would start flaring like crazy and charging around the tank. i had to remove them in the end as i didnt want to stress him out. his tank is on my desk in my study. he comes to watch what im doing and if he dosnt approve will start flaring at me. which is almost daily. hes a real grump.