You will have to consult with a practitioner locally who is familiar with your case to get appropriate dosage information. Chinese Medicine is -not- Western Medicine in any way shape or form so linear models of conditions=dosage or weight=dosage do not apply as well. A 300lb patient might need 5 tablets once a day with some other formulas and a child might need 30/day depending on their condition. There are very few generalizations in Chinese Medicine that are useful.
The only definitive thing I can say is, yes, you should not take any formulas besides cold/flu formulas (largely) when you are sick and most formulas that move the blood or resolve stagnation, like xiao yao wan, should be avoided during menstruation to avoid increases in the menstrual flow (in most cases, some it would be preferred even or possibly even only used then however).
Most of the basic formulas like xiao yao wan can be taken on an empty stomach or with food as they are fairly balanced formulas. If people experience mild nausea with the formula on an empty stomach then some food with it is helpful - or the formula is wrong for them. Again, these are things that a practitioner can help with.
Most herbal formulas are meant to actually resolve the underlying causes/symptoms so while -some- formulas are safe for very long term use 1-2 years or more, some are really intended to only be used for a few weeks to a few months and then changed to something that is more appropriate to the current issues. Chinese Herbal Medicine is far more of a moving target than the western medicine=symptoms=management model.