In most cases, ethical vegetarians do not use intelligence as a deciding factor in whether something is a moral candidate. Someone is not more or less worthy of ethical treatment based on how smart they are.

Utilitarians strive to maximize utility/happiness, which in many cases takes the negative form of reducing/eliminating suffering. Capacity to suffer is the one shared thing which is non-trivial in ethical decisions. The place intelligence comes into play is mostly in violating something's will, which causes psychological/mental suffering. It's tricky though, because if you view will as simply danger avoidance, you'd be hard pressed not to claim that fungi and plants have a 'will'.

edit: re: pescetarians. This is true but in reality it's a gradient of varying levels. Some vegetarians will use leather products, insecticides and other things. Some vegetarians are strictly doing it for health reasons. In that case, pescetarian is probably the healthiest diet and makes the most sense in that regard.