This thread is political in nature. It is not of much concern as to what weaponry the Su-27 shall be equipped with, or against which adversary this purchase has been planned(Mexico does not have any external threats). The Su-27s shall be equipped with standard equipment only.
The thread attempts to answer why did Mexico choose Russian equipment over US equipment, instead of US fighter jets despite being a "neighbour" of the US and one of its closest allies.
Mexico does not have ANY external or domestic threats. In fact I have thought numerous times as to why some Nordic nations equip themselves with the most sophisticated military hardware, when they are probably the most peaceful nations and have cordial relations amongst themselves which are very enviable by India/Pakistan/China/Iran nay, all of Asia and Africa.
Mexico only has Antonov aircraft from Russia. The most advanced fighter jets jets of Mexican AF are the F-5s.
It is thus imperative that advancement of their AF is due.
Nowadays there is a political issue in the US over granting of citizenship rights to illegal immigrants. Mexico shall be greatly affected by the outcome of the decision. It may have already anticipated that the decision in the US is unlikely to be in its favour (a significant proportion of Mexican foreign exchange is brought by remmitances from its immigrants in the US).
Thus, the decision to purchase only a small token number of Russian jets is likely to be a political means to lodge a protest with the US on this issue.
The numerical quantity or quality of the Su-27 will not threaten the US militarily in ANY manner which will thus help in maintaining the larger halo of the cordial and allying background with the US.