The best way to fight with a polybian legion is like this. First you must deploy your units in squares, each square represents one maniple. You need 4 maniples each of Hastati and Principe, and 3 of Triarii, there where always fewer Triarii then the other two. Now in a legion there normally there was 1200 each of Hastati and Principe and the same amount of Velites, so you will need 4. In a standard legion there was about 300 cavalry, I usally take two or one, as there is always the Consular bodyguard. Fill the remaining option with Accensi, there are worth it.

Now, when you go to battle deploy your legions in a checkboard formation, Hastati in front, Principe behind and triarii the third line. Leave the Accensi behind the Triarii. Deploy the velites between the maniples of hastati, and deactivate the skirmish mode. When you attack, shower your enemies with javlins, then attack first with Hastati then the Principe. If you are attacked take a defensive approach. Wait for the enemy to be within javlin range, shower them, then pull back the velites out of harm way and fill in the gaps with the Principe. Only use the Triarii to help in desperate situations, and try to outflank with the cavalry. If you what to add some italian allies replace the Hastati with the Hastati Samnitici and Principe with the samnites milites. sadly there are no replacments for the Triarii, but the Classic italian Hoplites will do.

Cheers.