What is the difference between stirring the egg yolks and sugar then adding the heated cream and heating the cream and sugar together then add the mixture to beaten egg yolks? Also some recipes call for egg yolks only and some egg yolks plus whole eggs. What is the difference?
In this case, the mixing order won't make that much of a difference. You need the sugar dissolved and the yolks broken up.
As for using just yolks vs yolks and whites.
Yolks have a lot of protein but they are very high in fat. Whites are just blobs of protein, of course. Using only yolks creates custards that are very rich and creamy but also high in fat (which is where "rich and creamy" comes from). Using the whites gives some of the needed binding power while reducing the fat a little.