Where does it say anything about them guarding God's throne?:
Isaiah 6:1-3 "In the year that king Uzziah died I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple. Above it stood the seraphims: each one had six wings; with twain he covered his face, and with twain he covered his feet, and with twain he did fly. And one cried unto another, and said, Holy, holy, holy, is the LORD of hosts: the whole earth is full of his glory."
The Seraphims merely stand above the throne 'worshiping' God. They seem to cover their faces from the brightness of the Glory of God, sort of like Moses in Exodus 3:6, which is notable because the meaning of seraphim carries the notion of burning, making the brightness of God all the more significant when some sort of fiery beings have to cover their faces in his presence. and regarding their feet consider these commentaries:
Barnes - "He covered his feet - In a similar description of the cherubim in Eze_1:11, it is said that they covered “their bodies.” In Isaiah, the expression clearly denotes not the feet only, but the lower extremities. This was also an expression of reverence drawn from our conceptions of propriety. The seraphim stood covered, or as if “concealing themselves” as much as possible, in token of their nothingness and unworthiness in the presence of the Holy One."
Gill - "with twain he covered his feet; or body, that it might not be seen, as the Targum; as conscious of the imperfection of their conduct, walk, and conversation, as ministers and Christians, in the sight of God, however beautiful their feet may appear to others, Isa_52:7,"