ICO does takes LORs from TAs, or at least they did with me. But, the stipulation is that the letter has to be signed by the professor in charge of the class/lab. I would suggest emailing them just to double check that the policy hasn't changed.
I sent in a letter of recc from a TA to ICO (not co-signed by the professor, he only signs it if u get at least an A-, I got a B+) ICO didnt say anything, but then I also had a pre health committe letter and it also included letters from professors.