The ESP Guitar Company

hello, i recently bought an esp ex260. yay, my first esp guitar. well, im having a little trouble with it. i bought it used at guitar center and it played really well. i noticed it had been strung wrong, two of the strings were mis matched so i replaced them with daddarios 9's it seemed to have a slight ring when i pushed down on the string when reguarly playing. i read somewhere to put bigger gauge strings on them, all i had were some 11's but they seem really tight, like almost impossible to do hammer ons and stuff. i play in drop d tuning d a d g b e. im not sure what was on them i think it was 10's will putting 10's on the guitar solve my problem? shouldn't you be able to just use any gauge strings you want? please help im confused.