Slight issue here. The reduced cost of 16 points per base applies to a complete base of spearmen, so where you have 50% spear bases and 50% bow bases. If you have 25% spear then the front bases are mixed and the higher cost of 18 points applies to these (see final paragraph, page 4 of Dark Age Armies). I think this odd situation arises because unusually, the archers retain their shield.
Yes you can use Arab and Indian allies together, as Mahmud usually did.
"No one ever achieved anything without making a few mistakes along the way"