Though I havent tested it myself, from what i can remember those who have tried this before decided that adding capitalships to pirate raids isn't possible. My guess as to why would be that the Raid can't handle capitalship entities, it would probably work if you created a frigate entity out of the capitalship, though obviously this would remove levelling.
It's possible.
I've made a pirate MOD before, adding Captialships, all worked well.
Here is a screenshots with Captialships Pirate Raid.

The reason I didn't publish it was another problem.
After these Captialships & Frigates finished raid, they return to the Pirate Base and stay.... so, here is a screenshot of many returned raid groups...it's horrible!!!

There were near 200 Captialships and over 200 seigeships and near 100 LRM and over 100 squadrons and other support ships.