Well since there's exactly two spaces between Charizard and Kingdra, they could literally be Vaporeon and Flareon haha, you're right it could be rough with Jolteon thoVaporeon is out of the running due to the way CHRs are structured in the set, Octillery is #191 while Frosmoth is #192.
I’m not 100% sold on Jolteon getting a CHR due to how many regular Pokemon are sandwiched between Frosmoth and Flaafy (5 if you exclude Basics) but Flareon has a really good chance when you compare how many regular Pokemon are wedged between Charizard and Kingdra (2 if you exclude pre-evos).