Or search: venial, forgivable, pardonable, justifiable
3 Syllables: SUU
beautiful, crucible, funeral, Juvenal, juvenile, movable, moveable, mutable, numeral, suitable, tubercle, usable, usual
4 Syllables: USUU
deducible, immovable, immoveable, immutable, inscrutable, reducible, removable, renewable, unsuitable
5 Syllables: UUSUU
constitutional, indisputable, inexcusable, institutional, irreducible, irrefutable, longitudinal
6 Syllables: UUUSUU
unconstitutional

Found 30 words across 4 groupings. Skipped 2 excess words of a similar stem.