You CAN have heros in the deathstar...atleast on g.conquest mode. I know this is true because Ive had the emporer, Tarkin, and Veers in the deathstar before..Also, you cant *create* victory star destryoers with the death star. Meaning: it isnt like skirmish mode where you can *make* new units during the battle. BUT at the beginning of each battle you are given 2 Victories and 2 Tartans as free units that do not count against the population count, leaving you completely free to bring in 5 star destroyers
(i sure hope thats right, i've been fixated on the rebels recently and not to sure.)
Oh, and if you have tarkin or someone like that, they will usually sit inside your biggest ship. So if you have 2 Acc, 1 Vic, and 1 ISD than they would be in the ISD....Again, im not sure if this applies to campain mode or not.