Whats the rule when trimming a mixed breed dog?

There are no rules.  Terrier and Spaniel breeds have similar trims a lot of the "breed standard" trimming can be incorporated into a mixed breed to bring out the particular mix of the dog.  Very popular at the moment are

Cavachons = Cavalier x Bichon

Cavapoo = Cavalier x Poodle

Yorkiepoo = Yorkie x Poodle

Labradoodle = Lab x Poodle

Cockerpoo - Cocker x Poodle

We can discuss all the options on meeting you as the options are endless.  Things to consider are face, ears, body, legs.  Think of each part and how you would like them to look, short body fluffy legs, clean poodle face, beard, moustash then we can create your own destinctive look based on what will suit your dog - a tailor made, unique haircut.