Does it matter whether Bactria would use phalanx warfare? I would speculate that exposure to the Roman Empire, plus a probable and perhaps inevitable period of stale tactical thinking and weak leadership which besets all states at some point in their existence means that eventually, as OTL, the phalanxes are going to take some beatings from lighter-based armies (probably the Romans but potentially anyone around them under the right circumstances) and will in consequence either partially or fully abandon the phalanx for the more sword-and-shield basis of the preeminent states of the Roman and post-Roman era. By the year 700 I expect that all memory of the phalanxes in Bactria will be entirely confined to perhaps long-forgotten national archives and chronicles, just as you can't, say, expect the Bactrian elite to still be speaking Greek to each other after 1,000 years (though I admit that it's quite possible for other quirks of Greek society to have become traditional throughout the Bactrian population).