Tim, that's a magnificent and very realistic story and battle scene! You've got quite a good number of men so that it's epic in scope, yet not so many that it takes forever to position them. Your handpicked German klickies look quite good and you've got a nice mix of Roman troop types.
Almost all the sources I've read have said that centurions fought at the far right of the front line and this is why their casualty rate was so high. As Tim shows, the optio is in the rear with his staff to urge the men forward and to make sure the men don't flee. From the rear he also could receive orders from the commander's runners and take them to the centurion at the front or, if the centurion was killed, replace him.
Scholars can't fully conclude and/or don't fully agree on where signifiers stood during the battle. Some possibilities include the following: in the center of the front rank, in one of the three spots nearest the centurion, in the middle of the second or third rank, in the middle of the century, or in the rear, either in the middle, the far left, or the far right. All these positions make sense depending on your point of view, and there's a good chance he was placed in different positions depending upon the seriousness of the situation. That said, being in the front rank sounds suicidal and thus, in my opinion, not probable.
As for the cornicen, it's also a matter debate where he would stand. I've read that he would either be in one of the three spots nearest the centurion so he could sound the centurion's orders or at the middle rear of the century to sound the orders from the commander. To me, the latter sounds the most probable.