Do Sheikhs from the Al Nahyan family marry outside their families or other ruling families from their country?
Yes they do and did, albeit less frequently than, for example, by the ruling family of Dubai.
Are they allowed to marry with foreigners that arent noble blood?
Yes, they allowed to marry outside of ruling family.
For a clearer understanding - past meets present: Here in UAE being a Shaikh means to occupy a special position within a tribe. The tribe system was (and often is) the mainstray of the social and political life in our region. The families were moulded into tribal groups for their mutual protection and well-being and governed by traditions and cultural heritage. This traditional society helped to formulate and elaborate an complex system through the formation of social units that began with family and ended with tribe. These corporate units had and have a traditional leadership. Every clan, tribe or alliance of several tribes was and is led by a Shaikh. In a sence he was "elected" by his tribe and as such is a symbol of people regardless of a noble ancestry.
A Shaikh had and has to steer the fortunes of a tribe - and now that of a modern state - and for the sake of his people successfully through ruling experience and family tradition. Basic requirement always been that a Shaikh has a firm belief in the values of consultation and consensus. His judgements were distinguished by their accurate insights, wisdom and fairness. The prestige of the tribe among other tribes depended on the Shaikh's conduct and manners. He had to be acceptable to remain in power. Shaikhs maintained their authority only insofar as they were able to retain the loyality and support of their people, in essence a form of direct democracy, though without the paraphernalia of western forms of suffrage. If a Shaikh did not act according to these traditions and values, he could be deposed in the past.
However this position was and is not automatically passed from father to the oldest son. Often the best man in the family would inherit the role. He was (and is) chosen on the basis of a combination of popular assent and personal prowess, that included his ability, wisdom and natural powers of leadership. The decisive factors are also forethought, consultation with his people and willingness to build consensus coupled with experience in mediation. A Shaikh have an obligation to feast his fellows periodically - on occassions, visits and celebrations. He also has a duty to be compassionate and generous, giving to those of his people who may be in need. These activities did not widen the Shaikhs influence; they were merely expected of one who was, and still is, seen as a father to his people.
The chief of one of the more powerful clans was chosen as the paramount Shaikh of the whole tribe and one family could hold this positions for serveral years. By historical events and founding the state in 1971 it came about that certain families with such a long family history as paramount Shaikhs were among the six ruling families of the Emirates. A Sheikh, however influencial, remain an equal among equals for the tribesmen.
There is Sheikh Khalid bin Saeed Al Nahyan who married Sheikha Shaikha Al Khalifa as his 2nd wife in July 2011. Together they have 2 daughters Sheema (2012) and Alyazia (2014). She is 20 and he is in his late 30s, so it is commom in the UAE to marry older men? And this Sheikh divorced his 1st wife who was from the Al Nahyan family before marrying Sheikha Al Khalifa. So does it make her 2nd or his 1st wife?
It is common in UAE to marry regardless of the age difference between spouses. So it is quite possible to marry an older man; or even being possible that a woman marrying a younger man. There's no rules regarding possible age differences. If a Shaikh (or someone else) marry a woman and get divorced and married again then the wife of his second marriage will be his "first wife" (like first lady). If a man has several wives at the same time, then between the wives there is actually no particular order (unless you look up to the date of their wedding).