I've found that missile units behind the Town Watch are useful. The Velites I have only used as skirmishers and to draw the enemy into a trap, if I've been clever enough to set one up. I like to protect my archer unit as much as I can so I can use it during seiges when in Gaul. I haven't used wardogs much, but from what I've heard they are very useful against light infantry and cavalry, especially in pursuit.