@maotd Assumed you're complete F2P, the safest bet to get rainbow would be 2017 gacha and slowly reck up rainbow coins, but without priority you might get girls that not in your spec. Excluding some broken stages, just gold with 2-3 dupes would be more than enough. Otherwise just overload yourself with FG, like 1000+ and let's hope 0.5percent is true.
TLDR: I think it just better to go for waifu/lovers you want.