I don't have the money right now to equip all of my talitot katan with techelet, but I'd like to put it on my Talit Gadol (Please, no discussion of Techelet itself - I've made up my mind on that one). Is is permissible to "mix and match" as it were?
I recommend that you discuss your own situation and best course of action with your rabbi.
I posed this question, regarding my own entry into the techeilet world, to Rabbi Jack Bieler in Silver Spring. If I recall correctly, he told me that if I only felt I could afford one set of strings, I should go ahead and put them on one garment, and that whether that would be the talit katan or talit gadol would be essentially my choice. His rationale was that fulfilling this mitzva with one garment is better than fulfilling it with none.