IFRA has some standards (under "standards") that relate to some potentially toxic fragrance ingredients in cosmetics that are either to be used in small quantities or are prohibited under IFRA standards. If it were me, I would avoid these altogether and they are probably not all that easy for non-professionals to buy anyhow. I would just ask whomever you are buying the oils from about any problems with the ingredients, including any potential allergens. If they are reputable dealers, they won't be selling anything you would need to worry about without warning you. Otherwise, the ratio of oils to alcohol depends on what strength of fragrance you are creating, i.e., eau de parfum, eau de toilette, eau de cologne. Short of outright poisoning someone, you are probably not legally bound by IFRA guidelines as long as you aren't selling these fragrances. I'm sure there are reference books for this but unfortunately I don't know what they are.