AFAIK the two licences are considered completely seperate.
I hold both an ATPL(A) and an, admittedly now lapsed IR, ATPL(H) with 9000 hrs on the A and 4500 on the H. Trying to keep them running concurrently was a nightmare as the requirements of employers clashed at every turn and the restrictions on the hours available in a year caused alot of problems.
For the purposes of the exams and thus qualifications the CAA under JAR have an ATPL(H)-(A) bridging exam and, again I haven't checked, I believe the same exists for (A)-(H) with a differences exam.
The requirements for the issue of the ATPL(H) are the same as the CPL(H) but with an IR.
Be aware that many employers will demand both a type rating and an IR which, if you are looking at S76 or A109, is pretty expensive.
300 hrs rotary would get you, I think, CPL(H) but check with a proper school.
Good luck.