I usually think of hedging in terms of beta exposure, and generally this beta exposure is to the market. Lesson 4 in the lecture series talks about beta hedging, and Lesson 13 discusses beta exposure in general.
A long-short strategy with a large number of securities in each leg is very likely going to be fully hedged. My reasoning for this is that if you sample n securities, the chance that the beta of that sample is not 1.0 decreases as n grows larger. So by the time you're sampling 500 securities for one leg, the chance that that leg's beta is not 1.0 is low. This is, of course, assuming that you don't have a sampling bias built into your ranking system.
Because both leg's betas are likely to be 1.0, running a portfolio of both will yield a beta of 1.0 - 1.0 = 0. Again, if your ranking system introduces a weird bias in which the longs have lower or higher beta exposure, then you may need to correct for this to achieve low beta. That said, the ranking system may introduce say a 0.8 beta leg in the top and a 1.0 leg on the bottom, I would be very surprised if you managed to rank such that the beta spread was very large. Consequently, I suspect you'll still end up with low beta (0.8 - 1.0 = -0.2). Lastly, beta exposure can be to anything. And of course your beta exposure to your ranking factor will be 1.0. The important thing is deciding the factors to which you want no exposure. The market is usually one of those, because it's quite volatile.
If you do find your long-short strategy to not be very well hedged in practice, I suspect you have a strong bias in your ranking system. Approaches to correct this might include re-evaluating your ranking system as discussed in Lesson 18, and weighting your legs differently. If you find that in practice the beta of the long leg is b1, and the beta of the short is b2, then you can re-weight your legs by solving a system of equations:
w1 * b1 + w2 * b2 = 0.0
w1 + w2 = 1.0
Of course, this is assuming that you can predict future betas. The historical betas would have to be pretty consistent for this to work. Lessons 7 and 10 go into estimate instability more.
Hope this helps, I'm around if you have further questions.