Electrons move spontaneously towards those compounds with the more positive redox potentials. The standard for the whole scale is pure hydrogen gas at 1 atmosphere pressure in contact with a platinised platinum electrode immersed in 1 molar acid. This is a bit inconvenient for biological systems, so we often work with Eo' which is the redox potential measured at pH 7. When everything is present in equimolar amounts, a good reducing couple can reduce a weaker couple. However, a weak reducing agent could reduce a stonger reducing agent, providing that the weaker couple were largely in the reduced form, and the stronger couple largely in the oxidised form.
Similar considerations apply to oxidising agents, with the argument appropriately inverted.
A little care is needed with the arithmetical signs in deciding which number should be subtracted from what. Remember that G is negative if the overall reaction is favourable. At key points along the chain, the difference in redox potential between adjacent carriers provides the driving force to pump protons out of the matrix space and into the cytosol as part of the overall energy coupling mechanism. When the system reaches equilibrium, the tendency for the diffusible ion to escape through the membrane, down its concentration gradient, is exactly balanced by the electrical force attracting it in the opposite direction. The membrane potential changes sign if you reverse the orientation of the gradient, or if you substitute a diffusible anion for a diffusible cation keeping all the concentrations unchanged.