"(a) Except as provided in paragraphs (b), (c), and (d) of this section, a person who is applying for an airline transport pilot certificate with an airplane category and class rating must have at least 1,500 hours of total time as a pilot that includes at least:

(4) 75 hours of instrument flight time, in actual or simulated instrument conditions, subject to the following"

Basically, this says that your instrument time must be "Time as a pilot", but does specify category or class. Your instrument time should all count.