IMO I think a beginner mistake is to be too eager to repot trees after purchase. In the big scheme of things and after decades of growing and repotting trees, I can safely say that the number one factor to successful bonsai is not in the soil or repotting. Long term care is dependent on it but short term care will do just fine for a year in whatever it's sold in. Better to understand the plant and its growing needs than to shock the shit out of it by repotting out of season. Juniper is a plant that for all intents and purposes prefers a liberal portion of plain old sand in its mix. Being in Mass. you can probably get away with a more open and free flowing soil for your juniper but in many parts of the west and the south a finer soil is good for junipers and their tight fine root balls. Coarse open soils allow almost too much air exchange and roots can dry out.
Just remember the tree has millions of years in knowing what to do, you not so much. Sometimes helping just speeds up a problem where none existed before.
