Or search: robust, energetic
2 Syllables: SU
Nicholas
3 Syllables: SUU
bitterest, bitterness, frivolous, limitless, picturesque, rigorous, Spiritus, syllabus, timorous, villainous
4 Syllables: USUU
antithesis, carnivorous, felicitous, indigenous, solicitous, vociferous
5 Syllables: UUSUU
abolitionist

Found 18 words across 4 groupings. Skipped 1 excess word of a similar stem.