Can the Atari 7800 games be played on the Atari 2600

Of course not, that makes as much sense as PS3 games working on a PSOne. Backwards compatability can only do so much. Think about it; if 7800 games would work on your old 2600, why would you need a 7800 in the first place? But in relation to your question, 2600 games are compatible with the 7800 console.