May sound like a silly question but I wanted to get sure that I can use Captain Terro and a Dewback Rider at the same time in a campaign mission.

You are limited to the number of deployment cards in the expansion, so you can have both Captain Terro and a Dewback Rider.

You can't use two (or more) Dewback Riders in the campaign though. Terro being unique also means only one of him even if you have two deployment cards for him.

In a campaign, players are limited to the number of Deployment
cards included in this expansion. Therefore, the Imperial player
is limited to one Dewback Rider group, regardless of how many
copies of this expansion he owns.

I'm wondering how this can be done if you only bought the big box. Doesn't it come with just 1 cardboard token but both cards?

I'm wondering how this can be done if you only bought the big box. Doesn't it come with just 1 cardboard token but both cards?

No, the big box only has the Captain Terro deployment card