I liked your idea of using part of the skirmish/push maps for firefight, but unfortunately it does not work.
The maps haven't been planned out for comp firefight, it really does play like a random area with 3 objectives dropped.
The only map worth using a sniper rifle on is crossing.
There are different roles you want people to play, but the maps don't give you a position to play the role.

look at Counter-Strike (I'm not saying copy them).
After Dust2 most maps pretty much were a copy of the Dust2 blue print that worked for their game mode. 2 open areas for the objectives, a middle ground, 2 ways to an objective area etc...
The design gave the players a position to play and a "role/weapon" to use when playing that position. which allows people to perfect and get better.

Even in competitive sports like soccer or basketball, you have strict rules and everyone has a role to play, based on the rules of the game and the size of the court/pitch.

Your maps need to be designed in a way that promotes "role" playing and fits the 3 objective firefight mode that you have. that way people can play competitively and strive to get better at the roles and areas of the map. After all this is what Teamwork is.

I'd recommend just making new smaller maps for competitive.

I hope this doesn't sound like a rant, It's suppose to be a negative, but constructive post.