I always said harachamon hu yakim only on chol hamoed sukkos but not on yom tov as this is what I was taught, is there any mekor to this or is it incorrect?
The general minhag in Klal Yisroel is indeed to say it even on Yom Tov, however the Chasam Sofer didn’t say it on Yom Tov, but only on Chol Hamoed. There are a few reasons given for this. See sources.
Pri Megadim Eshel Avraham 490-1, Eliyahu Rabba 490-4, Luach Eretz Yisroel Minhagei Chasam Sofer chap 8 ftnt. 11. See Sefer Yismach Yisroel ftnt. 1534 who brings from Kovetz Mivakshei Torah vol. 35 pg. 37 why the Chasam Sofer did this. However, the general minhag is to say it even on Yom Tov.