When I do my BC transfers, I use a medium - thick for the boarder and any lines you want to serperate colors and tehn I use a thin-medium to fill in the colors, if it is too thick it comes out looking lumpy and not smooth looking.
Good luck and make sure you post pics good or bad for us to see
I saw on the article regarding buttercream transfer that you used a wilton black icing to trace with. Is that already in a tube? I have never looked for pre-made icing in that form on the "cake" isle at Michaels? Is that what you used?