A good tactics primer for ADLG is Julian guide to ADLGv3.pdf, which can be downloaded from the ADLG web site.
Julian's guide breaks the troop types down to
The combat factors for different troop types range from 0 to +2, with most combat modifiers (eg. disrupted, attacking a flank or rear, impact) being +/- 1. However getting a non-light unit on to attack the flank or rear of an enemy whilst it is engaged to the front inflicts an automatic hit and reduces the enemy base factor to 0. This suggests that being able to out manoeuvre and out flank your opponent is more important than having better troops.
As a new player, I would often go with two or (with Romans) even three corps with identical or near identical composition. However this means that at deployment time your options are reduced. For example, rather than two legions with one MI each it would be better to have one legion with no extras and the other with two MI.
With respect to specific unit types, I have found Javelinmen difficult to use. They are effective against LI and LH, except if the LI is in difficult terrain. However the big advantage about LI if being able to interpenetrate other units. This means that you can use them between your own and the enemy battle lines, whereas if you were going to use Javelinmen there you would need to leave gaps in your battle line for them to be able to evade through. Given that light infantry are only 4 points and Javelinmen are 7 points, light infantry are nearly always the better choice.
I normally use a couple of light infantry units per infantry corps. LI bow have a greater shooting range, but LI javelin get +1 in the first turn of combat. If you are using your light infantry as a combat unit in rough or difficult terrain the LI javelin is a better choice.
I have mixed success with elephants. Elephants are good against mounted or armoured opponents. Against other infantry opponents they are ordinary (+1 and ignore armour). There appears to be synergy between elephants and medium infantry, they have the same movement rates and the elephants mitigate the MI weakness against cavalry.
Because the maximum width for a group is 6 units, there is a tendency to plan your army corps 6 units wide. On the forums, they advise Using six of anything in a block is probably a mistake.