The NPA are 5th column insurgents fighting for marxist red china, if filipino, they are simply traitors, no different from enemy spies or commandos working behind enemy lines. The NPA was created by red china in the 1960's when the goal of china was global domination, after the spread of communism was contained in vietnam, china claimed they abandoned this goal of global communism, however they continue to support communist revolution in the philippines and many other countries. the marxist propaganda says china no longer controls or supports the CPP - NPA, after they were declared to be a terrorist organization, but like most propaganda, it is false. The NPA is similar to Vietcong insurgents that were funded, supplied, payrolled, trained, and directed by the Chinese Army to overtrhow the democratic government of Vietnam. The NPA - CPP works hand in hand with the CCP of red china, especially in regards to anti-US propaganda. The purpose of the NPA is to drive out investment, industry and employment, create economic crisis and poverty, which the CPP blames on the democratic government, no different than any other sabotuer. NPA units fight AFP soldiers no different than if they were Chinese Red Army regulars: the NPA kill AFP soldiers, disrupt local police, transportation, military industry, and pin down large numbers of AFP resources that otherwise would be used for national defense against external threats, such as China.
in WW1, the German 5th column of spies, traitors and sympathizers in France was so strong that the movements of french troops was very often heard on german radio, hours or days before the french line officers recieved the marching orders. the term 5th column was used to describe +100,000 strong, militant, uniformed, Nazi supporters in Czechoslovakia prior to WW2, that made the german invasion simple, and merged with the NAZI army after invasion. The NPA - CPP is no different, and support and fight side by side with thier Chinese Communist Party red army comrades.
The NPA is simply a branch of the Red Chinese Army for military invasion and subversion purposes. The NPA claims to oppose China on the spratly issue, but even this propaganda is useful to China, as the beligerant, hostile, militant, argumentative, confrontational posture of the NPA in it's phoney anti-china rhetoric provides an excuse for China to use force, while claiming it was in defense of filipino provocation, and not a first strike attack.
In the event of war, the MILF could indicate they would fight against China or the NPA, however it is unlikely they could be trusted to do so, as a MILF unit, and more likely they would simply hoarde weapons or ammo given them, to use later against the AFP. [ much like chinese communists refused to fight Japan in WW2, but instead hoarded weapons provided by the US to later overthrow the democratic government of China]. Individual MILF or Abu Sayef fighters could be outstanding proper soldiers within regular AFP units, if some sort of amnesty, or other incentives were given out to ensure loyalty.