I didn't really have a problem about Damar working
with Kira per se
, I just thought the attitude change was a bit too 180 degrees. You can force yourself to work with someone you hate for a greater good, and it's admirable in the character that he is able to do so. But you don't just stop
hating someone from one day to the next.
I thought it would have made him seem even more admirable, apart from being more convincing, if it had been underlined a bit more how hard it was for him to put his misgivings aside(like they did for Kira who had much the same feelings).
As for what duties exactly Damar had performed in his career, we don't know; but given how militaristic in general the Cardassians are, I'd take any bet their army expects every
serving soldier to come up to a certain standard of combat fitness regardless of his functions. This would make tactical sense in any case - you never know how many men you'll lose in any given encounter, anyone who's enlisted might be called on to perform combat duties at some time or another.