Is the を interchangeable with が in this case?
And if I say 私が総てを愛している does it mean 'I'm everything that I love'?
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愛する is a straightforward transitive verb. Therefore, が always marks a subject (i.e. who loves), and を always marks an object (i.e. who/what is loved). は, as a topic marker, usually marks a subject but sometimes marks an object (See: Why can は and を sometimes be used interchangeably?). As far as 愛する/愛している is used in a simple sentence like this, が and を are not interchangeable.
This is a simple sentence that means "I love everything." Since すべて ("everything") is an object, 私 is clearly the subject (and topic).
This means "I love everything" (with emphasized "I"). 私 is the subject, and すべて is the object, anyway. Usually a sentence that describes a fact like this uses は, but when you use が instead of は, it has an exhaustive-listing function.
"I'm everything that I love" translates to 私は私が愛するすべてである (what a narcissistic statement!). You need to be familiar with relative clauses to understand this sentence.
There are a few exceptional words and constructions which can take both が and を. Please see the following questions for details. (Note that these are not related to your usage of 愛する.)