I take it this is referring to Literature. Unit 3a requires the study of two linked texts, one of which must be Shakespeare and the other literary heritage of any genre. Other cultures prose is needed for Unit 1 external exam and cannot be used for Unit 3a. I'm away from home and don't have the CA tasks with me. If you intend to study how Blake creates contrast in tone in some of his poems you must link it to how Shakespeare creates contrast in tone in one of his plays. Iit sounds difficult to me but would be permitted if it fits with one of the CA task titles. As far as number of poems is concerned, it's that old grey area again! The requirement from QCDA is study of 15 poems but students only need to write about 2 or 3 of those. Absolutely fine to change range of poems and scope of questions for different ability groups, in fact it is to be recommended. Come back if this still isn't clear.