When creating a Mail Merge template for use with Redtail CRM, you can use the font you would like to use. You just need to make sure that you've set this on the template prior to uploading to Redtail CRM.
If you have done this, but the font is incorrect on the individual documents after you run your merge, you'll need to change the file type on the template. While you can't make this change within Redtail, you can download the template from within Redtail under Manage Your Account —> Mail Merge Templates. From there just right-click the template and select the option to download it. Then, save it as a .doc file rather than as a .docx file. You'll then need to delete the previous version from your CRM (to avoid confusion) and upload the .doc template.
Your font(s) should then display correctly after running your merge.