The worst that will happen is that your site stops working. In which case you just delete those lines, or the entire file, in .htaccess.
For Putty, the port shouldn’t be 80. Leaving it blank should work, but you can also try 22 which is default for SSH. You also need to go to the panel here and Edit your User to allow for Shell access.
If you want to do everything at your end, use SFTP, not FTP. Then you can download DreamHost’s default php.ini. As long as you’re using a clean text editor that doesn’t add DOS linefeeds, you’re in good shape. Filezilla does SFTP and has a built in text editor.