I think that they ought to dress like priests in western europe before the 16th century ideally.
I would try to discourage pom poms on the biretta, except as an honorary prize for being extra good.
Birretas (spelling?) before the 16th century were a bit different anyway...
Otherwise what they wear is up to the bishop.
Here's a picture of Pope John XXII in 1901 as a simple priest.
Here's a picture of an anglican minister (who may think he is a priest) in a canterbury cap from the same era.
I think the canterbury cap was the pre-reformation version of biretta in most of europe. Present day biretta evolved in the 17th and 19th c. to present shape.