It is good to add other countries just for the sake of adventure. After reading some threads on selling players I added the Russian leagues. Lo and behold, after not getting any job ops after taking lowly Grimsby up to nPower 1 from Bs, I decide to jump to the Russian league 1 when the Mordavia job opened up. I knew absolutely nothing about the Russian league but now find myself completely immersed in it.
The fact is, if you play FM to buy and sell big name players, then yeah, just stick to what you did, but if you bought it to test your managerial skills, load up as many as your computer can take. I have England, France, Germany, Scotland, Ireland, and Russia and my system is handling everything fine.