Very few battles (relatively speaking) were one army running into another, both deploying, and then fighting. Most were some sort of attacker moving in, finds the defender deployed or near to it, deploying himself, then the attacker tries to dislodge the defender.
Give the Romans a few hours to set up, and they'll have a covered ditch across their frontage with spikes lining the bottom, their flanks tied in with abatis, caltrops lining the other side of the ditch, and maybe even a turf wall to give the auxiliary archers and lgionary artillery (scorpions mostly) a better view of the target.
Conversely, if they are attacking and the Catalans defending, they'd have to neutralize those knights first and foremost. The four thousand light troops- spearmen with no missiles- they could probably safely ignore, given their tactics and discipline. Should the Almoghavars move in, three cohorts could probably send the lot of them running- first pilum shower, then gladius in the guts, step forward, rinse and repeat. The Romans would not crack, and if deployed in century wedges, would be hemming the spearmen in and breaking their formation. It would ebcome a contest as to who can hold out longer- and that is a contest the Romans could easily win. Okay, maybe four cohorts. It depends on the terrain and how they can keep the spearmen ahead and not be flanked.
The outcome would therefore not be decided by the Catalan infantry, but by the use of the Catalan knights. It is they who would ultimately decide the battle: if they can keep their mobility and use it wisely- and avoid casualties to either man or mount- they would have the edge. If they got tied up in melee and bogged down, they're toast.
[This message has been edited by Terikel Grayhair (edited 11-05-2012 @ 07:30 AM).]