Can I generate a merge for certain rows and not for others?

Yes, this is possible by using filters.

Access the filters settings from the crank wheel menu on the add-on side-bar

In the filter settings, you can add filters based on columns content, to: 
- select rows that will generate a merge
- and /or exclude rows that will not generate a merge

Warning: Do not delete or change the 2 first filters, see picture hereunder. These 2 filters are necessary for the application to work and generate merges only once.

To add new filters, select

1/ the Criteria Column: this is the column in which you want the selection of rows to happen

2/ the Operator and Row content: this is the condition to select or ignore the rows based on the column content.

3/ click the ADD button

4/ click the SAVE button 

Warning: Do not apply 2 filters on the same column, it will not work

In the following example, we have added 2 more filters: 

If a row contains the word Sheet in column training1 AND the word Form in column training2, a document will be generated for this row.

In the following example, we have added a column with Data validation and 1 more filter: 

Add your Data validation column from your spreadsheet menu: Data // Data validation

==> Create an in-cell drop down list