It is not correct that Nolvadex reduces levels of estrogen: rather, it blocks estrogen from estrogen receptors and, in those tissues where it is an antagonist, causes the receptor to do nothing.
The claim that Nolvadex reduces gains should not be taken too seriously. The fact is that any number of bodybuilders have made excellent gains while using Nolvadex. The belief that it reduces gains seems to stem from the fact that the scientific literature reports a slight reduction in IGF-1 (individuals using anabolic steroids were not studied though) from use of Nolvadex. Thus, Dan Duchaine reported that it reduces IGF-1 and therefore reduces gains. However, if this effect exists at all, it must be very minor, due to the excellent gains that many have made, and from the fact that no one has noticed any such thing from Clomid, which has the same activity profile.
However, I would not be surprised if one were to tell a steroid user that Clomid reduced his gains, he would immediately become afraid that Clomid reduced his gains (please note that no one I have ever heard of has noticed this.) Not having been so misled, however, he would not conclude this from his results. But if an authority publishes that such an effect occurs, whether it does or not it can become self-fulfilling by biasing the user.
Either nolvadex or clomid can be used on-cycle as anti-estrogens and post-cycle to help normalize testosterone production quickly. HCG can provide an excessive level of stimulation to the testes, which has the ability to shock them out of a long state of inactivity. Generally HCG is not necessary in small cycles where dosages are kept low. However, it will help normalize things faster than if it is not used, so many choose to run it on-cycle for this added benefit.
A standard dosage of nolvadex on cycle as an anti-estrogen would be 10-30mg a day. The amount used would depend on what effect the user would like to elicit. More would clearly elicit a stronger effect, but users should determine how much they will require by starting at lower dosages and working their way up, so as not to use more than they require for the desired effect. For those steroid users who are using highly aromatizing substances (such as testosterone suspension), or those who have a sensitivity to estrogen, using nolvadex during cycle will probably be a smart idea to start early in the cycle.