If we are talking about the undocumented/illegal immigrants, then Mexicans make up about 60% of that, and there is no reason not to think it will be the Mexicans who will make up the vast majority of these new immigrants under this plan.
Wrong, there are plenty of reasons to believe that a majority of new immigrants will not be Mexicans under the Senate's plan. The plan is an attempt to more or less try and put the rest of the immigrants on a somewhat equal footing. For years one of the big criticisms of our current immigration mess is that the Mexicans effectively crowd out the other immigrants in the system. The reason the percentage of Mexicans under our present policies is relatively high is simply a matter of logistics. The other immigrants in other countries can't exploit the loopholes in our immigration policies as the Mexicans do simply because they are not fortunate enough to actually live in a country that shares a 2,000 mile border with the U.S.. Less Mexicans "clogging" the bottleneck into this country simply means more available spots for the rest of the people throughout the world.
With 6 billion plus people living on this planet there is absolutely no shortage of people wanting to come here. There are probably several hundred million people in mainland China alone that would really like to come to America if actually given some sort of chance to do so. And this is just one of 120 plus countries that we are talking about.
The total percentage of non Mexican illegals is in all likelihood higher than what you claim because there is no comprehensive way to actually verify which visa holders have actually go back and as opposed to the rest who simply decide to stay.