I have quickbooks plus australian version. 2 part question: Can I add a totally different company's books and records that is a US company in USD currency?

I would recommend to use QBO US version to manage your company's data in US. One QBO account is for one company file.

https://quickbooks.grsm.io/US

https://quickbooks.grsm.io/us-promo

Hi kate-l-lobb-gmai,

Thanks for posting in the Community! You can have multiple companies in one QuickBooks Online account or email address. Each QuickBooks Online company you create requires an additional subscription.

If your business and tax rates on your transactions should be in the US, you can sign up for the US version of QuickBooks Online. Let me guide you on how to do it. 

1. Go to https://quickbooks.intuit.com/choose-country/. 
2. Locate for the flag of United States (if business is in the US).
3. Choose the QuickBooks Online version you want to sign up.
4. If you would like to use the same email address on your US account, kindly scroll down and just click on Sign In button under the Adding a company to an existing account?.
5. Enter your login details and then click Sign In.

Once done, you can check or change the home currency on the created file by following these steps.

1. Go to Settings ⚙, then select Account and settings.
2. Select Advanced.
3. In the Currency section, select Edit ✎.
4. From the Home Currency ▼ dropdown, select your currency.

For more detailed information, you can also check on this article: Create or add another company file to QuickBooks Online.

Feel free to drop a comment below if you have any other questions. Have a great day!