Added at trace (CP) or after cook (HP)? (Old School method)
I think what @TheGecko is saying is that it doesn't matter if you add the avocado oil then or simply include it with the rest of the oils. The result is the same.
If you try the shea recipe, add it to the oils before adding the lye solution.
Doesn't matter what oil or butter you use when it comes to Cold Process because of the nature of the process. Because ALL of the oils/butters need to be combined prior to saponification, there is no way to determine which oils/butters are left unsaponified (without a chemical analysis)...you could have a little bit of every oil and butter left unsaponified.
With Hot Process it's different...saponification has already occurred so the lye is already used up.
I am assuming you are asking about adding shea butter or mango butter to HP soap after gell has occured & while the batter is still fluid enough, but also hot enough to melt the shea or mango butter. Is that what you want to know about? IME, mango melts easier that shea, so it seems it would be the better choice as long as the batter is still fluid enough. However, I would suggest you pre-melt the butter you want to use as the SF addition and incorporate the melted butter into the still fluid-ish HP batter, rather than to add a room temperature butter to a hotter fluid, as that could cause the pourable batter to cool too quickly prior to pouring.