Edit: in Part 1, I meant to say using some product that you apply when joining two dissimilar metals.

The anti-ox compound is necessary for most aluminum connections but it's NOT sufficient for making the connection itself (i.e., you can't just throw some anti-ox on it and then use a regular wirenut).

You need some connector that is specifically listed for AL-to-CU connections. Just want are you connecting the #6 copper to?

You could run #6 AL for a 30 Amp circuit, but why would you do so?

Note you can't tap the old circuit for two units if it doesn't have 50A ampacity (#6 aluminum depending on what sort of cable we're talking about may not have such).