Yes, you should follow Spiritual Satanism alone if you are to be a SS. Especially angels which are dangerous enemy beings, you should completely cut off all ties with them. They are evil enemy beings, enemies of Satan. We don't worship any Pagan Gods either, even though they are our Demons. Like it's been said in the site, Spiritual Satanism is unique in the sense that it doesn't mix. If you try to mix it with other things you'll cause yourself problems and negative experiences, especially if it involves enemy beings like angels. It's a big no-no, mixing it. You either are a full SS and nothing else, or not at all.
The site also says:
"NO GOD HAS A RIGHT TO INTERFERE IN MY AFFAIRS, AND I HAVE MADE IT AN IMPERATIVE RULE THAT EVERYONE SHALL REFRAIN FROM WORSHIPPING ALL GODS."
Those who worship other Gods end up confused and are open to all kinds of influences. It is important to stay focused. As Satanists, our God is Satan and there is no need to call upon other gods. Demons act as protectors and direct us to be strong in Satan. They can also act as mediators as they work with us closely, but we do not worship them. Satan does not demand or want any slavish worship.