Or search: hygienic, healthful
3 Syllables: SUU
chemical, clerical, ethical, festival, medical, metrical, plentiful, spherical, vesicles
4 Syllables: USUU
heretical, hysterical, identical, indelible, intestinal, nonsensical, numerical, poetical, symmetrical
5 Syllables: UUSUU
academical, alphabetical, antithetical, arithmetical, asymmetrical, biochemical, ecumenical, evangelical, geometrical, hypothetical, theoretical
6 Syllables: UUUSUU
incomprehensible, infinitesimal

Found 31 words across 4 groupings. Skipped 2 excess words of a similar stem.