Susie
I am a bit embarrassed to say that I use what is handy. I try to pot up cups to gallon size in the same mix, which is generally 50/50 peat/perlite. After that I go up to three or five gallon and use a potting soil. If I don't like the looks of it, I will add more perlite. I have used Promix with good results, but is sometimes tough to find around here, so I am now using Miracle Grown potting mix, again with good results. Mind you, it has fertilizer added, so I am hoping it will permit hardening off/ lignifying by the end of summer. I don't have so many (yet) that I need to buy bulk and if something doesn't work it is easy and inexpensive to change it. I have little, young trees and am getting figs from Negronne, Violette de Bordeaux, Hardy Chicago, Blue Celeste, Green Ischia, Troiano Calabrese and some unknowns, not a lot but given the size of the plants, I am calling it successful.
I have read others on the forum extol the virtues of various mixes, so I will nod to those with more knowledge than I on the matter.