I believe Hoosier Vapes recommends replacing the little "white doughnut" back into the cartomizer end after pre-filling the carto. This provides for a tighter seal with their specially designed top cap/mouthpiece. That is what I do with my Elegance tank's cartos.
This is the rubber or plastic piece in the end of cartomizers when they are shipped and we generally remove and toss away. (I'm not talking about the condom covers).
I have used both flanged (
smoktek) and unflanged (Ikenvape) cartos in my Elegance tank. Neither leak, so the flange is not the issue causing your leakage.
Although I haven't found the need to follow it, Hoosier Vapes also recommends a particular method of replacing the top cap after filling the tank to prevent flooding the carto. The technique is described at the bottom of the page where their tanks are sold. This involves
tilting the tank so that the air bubble within the tank matches up with the juice hole in the carto as the cap is applied to the tube.
It should also be noted that there have been a couple of instances reported here on the ECF forums where the Bella or Elegance tanks had minor enough imperfections which caused some leakage issues. Reporting these issues to Hoosier Vapes resolved the problems to the best of my knowledge.
There's a rather large thread on these tanks here on ECF:
http://www.e-cigarette-forum.com/forum/tanks/316886-show-us-your-bella-tank.html?highlight=lets+see+your+bella+tanks