Or search: mix, stuff, botany, mingle, nekton, commix, combine, unitise, multiple, plankton, totality, summation, congeries, phytology, collective
4 Syllables: SUUU
advocating, aggravating, agitating, allocating, animating, fascinating, graduating, irrigating, mitigating, navigating, propagating, vacillating
5 Syllables: USUUU
collaborating, congratulating, contaminating, elaborating, eradicating, evaporating, exaggerating, exasperating, investigating

Found 21 words across 2 groupings.