If you have made your decision to build your ultimate team by the players from either same league or same nation, in fact there are some other factors still that you should take into consideration to ensure that you could get up the team chemistry to 100. Unfortunately, the league or the nationality are just not enough.
It is very important to decide which formation to choose as it could have a big bearing on how many you will need to invest to get the team chemistry 100. As the indication, the 41212 formation card could sell for even between 6k to 12k FIFA 15 coins anywhere. If you have made your decision of which formation you will use, do the research. Once you have chosen the popular one, you should make sure that you have known how many the formation card is selling for just before you are searching for the players so you will not pay too many for the players in you preferred formations.
The players in right formations could give a healthy, nice boost to the team chemistry as well. You could get about 3 players away out of the formations but still get team chemistry 100 taking in account the other factors.
The players who play in the preferred positions will increase the team chemistry. It will be much better if you could get more players to their preferred position.The odd CDM playing CM or ST playing CF will not make too many differences while it helps. You will be able to get more players away with out of the formation if you have more players that play in correct positions.
Get great gold managers with the preferred formations! You are able to use the gold manager in silver and bronze teams as well. Choose the manager with same nationality as most players in the team. It would be straight forward for your team based on the nationality while maybe you will have the decision to build your team that is based on the league. Good managers with right nationality and formation will boost up the chemistry by even 20 points. It is one of the cheapest methods to boost the team chemistry by far.
As a summary, what you need to do to boost the chemistry are: choose the league or nation that you would like to base the team around; pick the players in the preferred positions and formations, keep in mind that you will be able to get away with some out of formation or position; Get good managers who match the formation with same nation as most of the players; a useful tip is buying the players with correct position and formation, which will allow people to purchase the players that are more expensive at cheaper price by choosing the one with unpopular position or formation and it will still be able to get team chemistry 100.