What categories would we use for investment transactions, dividends, outflows, fees?

I am not quite sure what you mean. Do you mean, that you just have a manual account that includes all your transactions for these four things? Or do you mean you have these four accounts in your Simplifi file, which would be normal. Also, just to make sure are you using Simplifi or Quicken classic?

If that is the case, you should create four accounts, one for each. You could then probably connect them to your banks.

As for investment transactions, if the account is not connected, you can create income categories for Dividends and an expense account for fees. There may already be some if you are using the built-in categories.

Anyhow, please give us more information in case I have misunderstood.