With regards to Broca and his amazing (and undeserved) rise to the rank of Legate and "Leader" of Cardassia prime, two thoughts strike me:
1) Broca was only promoted because every other Gul and Legate had either joined Damar's rebellion and/or had been killed by the traitor Gul Revok. Weyoun wanted another puppet to play with, and Broca is the archetypal creep who can kiss Arse with the worse of them. Which brings me to thought number two;
2) Broca owes his position to Dukat; this makes sense to me because of who Dukat is, namely a megalomaniac with a desire to be loved, such a person would surround himself with craven lickspittles who would agree with a praise his every move. After all Dukat's regime was based on very shaky foundations due to the execution of the leaders of the Cardassian Revolution of 2372 and most of what was left of the Central Command. Also, judging from the testimony (Sho'val?) of the late Ternkey Ghemor most of the Gul's where plotting to over throw Dukat.
As he already had one competent aide de camp in Corat Damar, Dukat didn't need to have too many competent enemies near to him, therefore Broca makes a useful barrier between Dukat and the rest of the CC.
I personally found Broca to be a total surprise to me when he first appeared in "Dogs of War" the man did not crime with any of the Cardassian's that we came to know and love. I wondered why they didn't make Gul Revok the new leader of the Union as he had destroyed Damar's Rebellion almost single handily!