Hi Steve,
I replied on the forum but alas it has gone down again. :-(
I've deliberately over-represented the triarii as the legion becomes too weak if they straddle two boxes. Also, a continuous line of triarii looks much better than a gappy line.
I don't believe later Roman armies had a line relief mechanism equivalent to the Polybian one- subsequent lines were fed into the battle but we don't know what happened to the first line; they don't appear to have been pulled back into a reserve that was capable of playing a part, again, later in the battle. There certainly wasn't anything like the "quincunx".
BTW I tightened up the wording of the line exchange yesterday, it is in a revised "rise of Rome" booklet.
Hi James!
Best, Simon