There is a reason for the order they are listed in:
the smaller a sin seems, the easier it is to miss it - and to think a sin has some sort of rights to venial status is mortal. Venial sins are venial not merely because of their form alone, but due to the slight premeditation, or unexpected temptation to commit them. On the other hand, to take time to justify a venial sin already committed, is to flippantly demand, over and against the fact of whatever slight the sin had caused you to pay GOD, that you need not feel so bad for it, since it was venial. If one comes around a corner a little carelessly, not heeding a quick flash in the mind to look before proceding, and he rams into an elderly lady and breaks her hip, hardly having had a moment to reflect on the thought for the missed precaution, so long as he feels badly for the hurt he caused, his sin is venial - but should he, just because he hadn’t intended in his carelessness to do quite this, place the fact that it was a venial sin in intent before concern that he in fact caused another to suffer for a degree of carelessness, then that attitude makes him guilty of a weightier sin. To insist rather on the degree of lightness of a sin, rather than take responsibility for the full weight of what Love was lacked in sinning, is to act very strange to the SPIRIT OF GOD, and to commit a mortal affrontery to GOD.
Thus, it is useful to be most dilligent about all seven deadly sins, from least to greatest.