Or search: thinkable, imaginable
3 Syllables: SUU
feasible, miracle, peaceable, readable, reasonable, regional, seasonal
3 Syllables: UUS
agreeable, foreseeable
4 Syllables: USUU
amenable, appreciable, procedural, satirical, unreasonable, unspeakable
4 Syllables: UUUS
disagreeable
5 Syllables: UUSUU
inconceivable, irremediable, irretrievable, unbelievable, unimpeachable

Found 21 words across 5 groupings. Skipped 1 excess word of a similar stem.