Can I use Classes AND Projects at the same time. For example, can I use Projects within a particular Class but not all?

Thanks for visiting the Community, @cmc27. Yes, you can use classes and projects at the same time by adding transactions (invoice, bills, estimates, etc. ) to the project. 

To make sure you're able to use the Class tracking and Project feature, you'll need to turn them on first in Quickbooks Online Plus, Advanced, and Accountant. Then, follow the steps to create projects, and start adding a class transaction to it. Here's how: 

Create your project

1. Go to the Projects menu, then select New Project.
2. Give your project a remarkable name.
3. Pick the customer you’re working for from the drop-down list, then select Save.

Add class transactions to a project 

1. Select + New, then, select the type of transaction you want to record.
2. Select the customer/vendor's project in the Customer/Vendor drop-down list. 
3. Add all other necessary details of the transaction.
4. Select + Add new in the Class drop-down list:

• To assign one to the entire invoice or receipt, select the class drop-down at the top.
• To assign one to each item (or row), add the class in the Class column (if you set one to each row in a transaction).

       5. Click Save and close.

Just in case, you can also create your budget by a project to get estimates.  

For future reference, read through this article to learn more about what else you can do about projects in QuickBooks Online: Projects FAQ.

Let me know if you have additional questions. I'll be looking forward to helping you more.