a poodle so long as properly socilized would make an absolutly fabulous service/therapy dog. they are eager to learn and please, they are a retrieving breed but if you get a puppy and from day one introduce him to the small animals and teach him they are not toys, you shouldnt have any problmes. Poodles are EXTREEMLY smart.
id go with a mini if your looking into poodles, toys tend to be a little more warey and a standard is bigger than you seem to be lookin into.
Doxies are not the best choice, they make great companions in terms of working but were bred to go to ground...meaning Chase small animals into their burrows, they are tenecious and known for facing down animals as feirce as badgers.
Bostons are less drivey than many of the other terriers but they are true terriers in every other way, if you can deal with a terrier personality a boston could work fine, again start from puppy and teach small animals not toys/food and never leave him alone unsupervised and it should be ok...
Pugs are a fun breed that thrive on being with their people, i can see a pug making a good service dog depending on the services your hoping the dog will provide.
Belive it or not, again depending on the services required chihuahuas can make great service dogs, Vixie alerts to seizures...she is trained to respond also but has decided she likes life better at home over working...they are generally good with small animals (not so good with insects which for some reason chis like to chase and eat lol)
of all the breeds, if your up for the grooming though a mini poodle would probably be perfect, smart, easy to train, eager to please, none shedding (good for service dogs as noone can claim "you cant bring it in here im allergic") like their people and generally enjoy having a job to do...
if you dont mind me asking...what kind of service dog are you looking for (services provided?)