How you pull this data depends on what action you want to trigger the entire FLO. For example, if you wanted to have the rows from the excel sheet to update the google Sheet every time there was an edit made to the Excel sheet, you would want your FLO to trigger on an Edit or Update type event. Then after qualifying the data that was received, you would read all rows of the excel sheet and then overwrite the corresponding rows in Google Sheets.
Part of the difficulty of migrating between sheets is something you mentioned, data integrity. If you look at the available Google Sheets cards, there is an option to download all Rows from a Google Sheet, but there is no way to upload a file into Google Sheet. Thus for your use case, the best way to handle this kind of read-write operation would be to give each of your rows in your Google Sheet a unique identifier that can be matched to a corresponding row in Excel. After qualifying the update performed in Excel you would generate a list of all of the rows of data from your Excel sheet, then use a list function to iterate over each row from excel to find its match in Google Sheets and overwrite all the data in that row, or if the row is not found then create a new row in Google Sheets.
I hope this wasn’t too wordy, but I am still having a hard time understanding exactly what you want done with your FLO.