Yes, that's correct; reduction applies to prepositions as if they were part of the next word, and there's even a limited number of preposition-noun combinations where the preposition steals stress: пóд ноги (but: под ногáми), нá руку (but: на рукé).
(I have to correct what I originally wrote after this, and double-checking is welcome.)
It does not apply to conjunctions. Но is always [nо]; а is always [a]; что is always [ʂto], etc. However, in colloquial speech, the compound conjunctions
то, чтобы and
то, как and their oblique forms can be pronounced as one word, with stress on the first element: ['toʂtə], ['toʂtəbɨ], ['tokək]. Потому что without the comma is always pronounced as one word.