Light Infantry Uses

[Titus]

Keep your Light Infantry to no more than 40 per unit as only 20 will be shooting missiles at any one time. Spread your LI across the front of your heavy infantry as a skirmish line to protect your HI from missile fire and chase away the enemy LI.
Once in missile fire range keep your LI stationary and issue a skirmish order (right click on the same hex as the unit is already located). This gives them 2 shots per turn and still allows them to escape if the enemy try to run them down.
Make sure your HI behind are no more than 50% stack value so your LI can run through when evading enemy contact. If your lights try to get out of the way of enemy cavalry and find they can not retreat because of the overstack rule then they will be destroyed. LI is no good against cavalry.
Gallic warriors are principally LMI so you need to use these as your main attack arm. They do not do well on a frontal attack against the Roman Legionary. Try to outflank the legionaries with your greater numbers. Remember to break your units down to about 90 - 100 in size to have many units to allow you to swarm round the legionary flanks. Gallic warriors also get an impetus on initial contact with the enemy until their fatigue is 10 or above so use fresh troops in combat when you can. Try to recycle the troops and give tired units chance to recover their fatigue. Always charge in to the Romans to maximise the kill rate. Keep your leaders close to the units but behind to avoid losses. Command control is more important than their contribution to combat.