When you look at countries that are predominantly male, such as China, Japan, etc., you'll notice that they are highly successful countries. My question is: Since Ceasars Legion is all-male, would they be better for New Vegas???
Your initial suggestion is rather flawed. Most countries with a substantial male bias in the population have it because of migrant workers - and the ability to attract lots of works occurs because of economic prosperity, it doesn't go the other direction. Nations with a substantial male bias among citizens usually treat this as a considerable problem as even in the short term it can result in a lot of strife and the emmigration of those who can afford it (who are often the better educated or wealthier).
There is even some very solid evidence that nations with large populations of unemployed or poorly employed young men often results in civil strife.
As for FONV, the Legion isn't all male, it's just that women are part of the slave caste. The Legion's ruthlessness may be helping them spread in the short term but this isn't tied to their gender balance.
I don't believe the Legion's structure can work in the long term. As they expand it will become more and more difficult for a top heavy government to operate properly. A more feudal system may develop but given the ruthlessness that has been fostered it won't take long before the various feudal lords start going after each other. As for gender, keeping women as part of a slave caste would likely cause problems down the road, although I think the Legion government would disintegrate before then.