Or search: arson, flame, firing, tumult, flaming, ignition, kindling, lighting, oxidation, oxidisation, oxidization, incineration, inflammation, incendiarism, deflagration
2 Syllables: SU
cousin, Covent, custom, dozen, Dutchman, husband, judgement, judgment, Justin, Muslim, muslin, oven, Prussian, roughened, ruffian, Russian, Tuscan
3 Syllables: USU
accustom, adjustment, Augustan, concussion, discussion, escutcheon, Etruscan, percussion, readjustment
4 Syllables: UUSU
repercussions, unaccustomed

Found 28 words across 3 groupings. Skipped 16 excess words of a similar stem.