Skirmishers were an integral part of the Roman Army, but in that case they were used against infantry, not archers. They were used to disturb the enemy lines before the Legions engaged. Then it would be nice if the Centurion aura affects the Skirmisher too. The Roman Skirmisher could also have a small bonus vs infantry to reflect their use, or a unique upgrade to the Elite Skirmisher, called Veletis or Auxilia, with the small bonus vs infantry, but another unique upgrade would be too much. Or they could just receive the bonus vs infantry without being affected by the Centurion. But I think the Centurion Aura affecting the Skirmisher is enough.
Velites was old republican era unit so it won’t fit as late western rome unit.
Maybe change the UT by removing the charge attack for centurion and give skirms bonis vs intanfry
DapperTrout1297 is correct. I did read about this. Auxilia is also not correct because it was composed by infantry, archers and cavalry. I think the legionaries themselves acted as Skirmishers in battle. In the Rome ad Bellum mod the Legionnaire can switch between melee and spear throwing.
Pilums, which were javelins, were used since the dawn of the Roman civ until aproximately the third century. They were then replaced by Plumbata, which were darts
The problem is this weird thing in AoE2 where the Skirmisher has a very low attack but with bonus against Archers and Pikes. I understand they wanted to make it a hard counter to Archers, but it was not necessary to add bonus damage. Only the high pierce armor would be enough. The Rattan Archer behaves exactly like this: a Archer-resistant Archer who is able to defeat most archers, with few exceptions.
The Skirmisher, considering that its cost stays the same, could double the base attack and move faster, and lose its bonus to Archers and Pikes. They could do more damage than archers, but with a lower attack rate so their DPM would be lower than archers. They would be perfect for hit and run tactics, while the archer is better for sustained fights. The Skirmisher would fire at a lower rate than now because they can carry only a few Javelins with them.
I think this because a direct spear or javelin hit looks very damaging, and could even pierce armor. But in AoE2 now you have Longswords with Gambesons being able to withstand 60 javelins and Berserkers that are not even damaged by one Skirmisher alone.
Skirmishers have a bonus against pikes because pikes need a counter in trash fights.