Google Apps Legacy Free Edition – Adding More Users

Short answer: Yes.

However, there are some things I think I should say about that approach.

For completely unrelated sites, it's probably best to sign up for completely unrelated Google Apps accounts. Domain aliases will work, but all sites will share the same limitations on how many users can be created (quotas). Separate accounts will each have their own independent quotas.

If you alias, you'll have 50 free users to divide among all the domains. If you create separate accounts, you'll have 50 free users per domain.

Hope that helps you. Personally, I'd create a new account.

No you can't do that and you will have to upgrade to the Google Apps for Business in order to get more space.

From the FAQ for Google Apps:

Can I have a subset of employees on Google Apps for Business and others on Google Apps?

Google Apps does not support Google Apps and Google Apps for Business user accounts as part of the same organization's Google Apps account.