Is it ok according to established practice to serve a salad in addition to crudités?
It is perfectly acceptable to serve both or just one or the other. Typically crudités are considered an appetizer but, when serving crudités, a salad is not always needed. I would base my decision on what type of gathering it is and the entirety of the menu.

From The Veggie Table:


  Crudités is a French term that refers to raw vegetables, so a platter of crudités typically consists of one or more types of vegetables served with one or more dips. These are great for parties and can be served as an appetizer, at a buffet, in place of a salad – the sky is the limit.


UPDATE - Based on the information provided in the comments, I agree with @Max - I would serve both crudités and salad.