Can arsenal play a high line with koscielny and Mertesacker?
the quick answer is yes, ive won the league repeatedly with arsenal playing a high line and kos and BFG (mertesacker) as my starting central defensive pair.
have a look at the guides on the SI GAMES tactics forum. there are three guides there on how to build a tactic, as well as a pairs and combinations guide, that will change the way you play the game. It becomes to much more satisfying once you develop your own tactic, rather than using others and not understanding why they are or arent working.
the second page of this thread, linked,
http://www.fm-base.co.uk/forum/football-manager-2014-tactics/152070-arsenal-possession-tactic-2.html, is the tactic i developed after reading those guides. The attacking and defending tactics in that thread have won me the league in the first year with, arsenal, stoke, southampton, nwecastle, chelsea, and manu.
there is not a single way to play the game, there are almost infinite ways, so dont listen to specific tactical proclamations, like the ones made in this thread. They key is to understand how you'd like your team to play first. Its like drawing. Artists dont just start drawing, they visualize before hand, same with golf, if you can see how you'd like to play, you are halfway there already.
I strongly urge you to go to the SI GAMES community forums and check out the top 3/4 guides in the tactics section.