It doesn't appear there are any light spears (or any light weapons which can braced against a charge at all). However, that doesn't necessarily mean you can't use the feat, you just need to modify your formation.
Most "spear/shield" walls are two men deep, at least. In this terrible, terrible diagram, T represents targeted or threatened squares, S represents the shield wall (heavy shield and let's say short swords), and R represents the spear wall (longspears all around).
Note that this formation is most effective against infantry charges. If you are expecting a cavalry charge, your best bet is to equip your front line with longspears, heavy shields, and short swords (or other light weapon of your choice). They should start combat with their longspears out, but after using them to counter the cavalry charge (which is likely conducted with lances), should drop the spears and switch to their shield and light weapon to maximize their melee survival.
Note that in either formation, the back row can carry shields and light weapons as well to switch to when front-liners go down.
Another worthwhile note is that armor spikes count as a light weapon, so anyone using spiked armor and a heavy shield can qualify for the Phalanx Fighting bonus regardless of their main weapon, by strict rules-as-written. (As a DM, I would probably not permit the bonus while they were actively attacking with a non-light weapon, but let it apply otherwise, so that someone equipped with spiked armor, a shortspear, and a heavy shield could gain the bonus if attacked by anyone who neither triggered a readied shortspear attack nor provoked an attack of opportunity that the character took with the shortspear.)