Is it worth reading Robert Service's biographies of Trotsky and Lenin, and what about Antony Beevor's "Russia: Revolution and Civil War"?
Secondly, any specific flaws that are present in these works and what sort of biases of the authors should I take into account before reading?
Thirdly, what books are the best and of superior quality to these books in their topics? (Russian Revolution and Civil War, biographies of Soviet Leaders etc)
Beevor for sure is a serious historian and worth reading. As for biases, for accomplished historians I have found it requires time and patience to read the subject from different historians to find what they include or omit. It just isn't easy to say, "Ah-ha! There is his bias and prejudice!" You really have to find the absence of overlap.
For further reading on the Russian Revolution and historical biographies, consider Simon Sebag Montefiore's "Stalin: Court of the Red Tsar" and his prequel, "Young Stalin". The latter of which includes the revolution.
Stephen Kotkin has a three volume work also about Stalin, with volume I primarily being a less a study on the revolution. The titular character simply being in orbit of the subject.