Can I add a field that auto completes data from Salesforce and then have fields auto populate based on it?

Sure - find out how

Item 1: Auto complete
Among FormTitan's special elements for creating a Salesforce form there is a special element called "Lookup". This fields, once configured by you can allow your fillers to search through Salesforce data and choose the right record. By entering only a few letters the lookup will open a list of records to help you complete the selection more easily. So in fact this field acts as an auto complete from Salesforce.
Read how to use the Lookup

Item 2: Auto populate
By adding a Salesforce Get integration, you can have your form fields auto filled in realtime from Salesforce - this is quite simple. 
Auto population can occur automatically on form load, or you can have it populate based on a certain entered data. The latter is exactly how you can combine the 2 items together harmoniously. 
Read about the Salesforce Get integration to auto populate fields
All you have to do, for example, is create a Lookup that contains your Salesforce Contacts names. Once a form filler enters a part of the name and presses on the "Search" icon all contacts containing this input will appear and he will only need to choose the right name from the list.
And now comes the fun part - when the name is chosen and the contact is therefore identified, the rest of the form fields will auto populate with more information about this contact.