Can I substitute a biga or poolish in recipes that call for a sourdough starter? If so, what should I take into account? Will the resulting dough require a shorter leavening time? If not, why?  I understand that substituting the sourdough starter with a biga or poolish won't give the same flavours to the dough, but I am not concerned about flavours. I am concerned only with the feasibility of the substitution without compromising the execution of a recipe. 
Yes, you can.  A biga or poolish is pretty similar to a sourdough starter, especially if the hydration is the same, and from what I've heard it is a 1-to-1 substitution.    I have seen sourdough starters that are made with commercial yeast - the extra time the starter has to establish itself, while being fed and discarded until it has settled, allows the right bacteria (lactobacilli, think the bacteria in fermented pickles) to join the yeasts and throw a party.  So the actual yeast isn't a problem, though the taste will be missing those elements the lactobacilli add.  From what I've read, I think sourdough has two main difference to the biga/poolish type of starter that you might find relevant.  One is that sourdough starters, having has longer to establish themselves, may have less gluten, etc in the flour of the starter (having eaten it all) - the starter itself may seem looser even if the hydration is the same.  Recipes may compensate for that difference, so the dough may behave a little differently.  Should still work, but just a little different - and bread doughs can already adapt to quite large differences, so a lot of tweaking may not be necessary.  The other difference that gets mentioned, is the timing of the rise.  I have heard that sourdough takes longer to rise, and I've heard that it is common to add a bit of extra yeast to a biga/poolish when making the dough to make sure the final rise [as compared to sourdough] stays on schedule.  These... are pretty much opposite each other, one suggesting biga/poolish is weaker and needs a bit of help (in the form of extra yeasties), and one suggesting starter is a bit weaker and needs a bit of help (in the form of extra time).  so I can't actually tell you which way to jump.    I would suggest just setting the timer for the rise a bit early, and from then check on it several times till it looks doubled or has the right amount of springiness or however you judge your proofing. 