Hey Dan,
Getting rid of the “All” button is easy – please use this CSS (you can paste it into the “Style: Add Custom CSS” tab of the Theme Options panel):
#portfolio-filter-item-all { display: none; }
Telling the page to filter the results by default is trickier – you will need to include some custom jQuery, something along the lines of this, which will need to go into the “header-functions.php” theme file. You will need to replace the “XX” with the name slug of the portfolio tag that you wish to use as the default filter.
Sincerely,
Andrii / Satori Studio