I'm not an expert, this is just what my doctor has told me and from research I've done, so I don't pretend to know everything. But it is my understanding that length of episode has nothing to do with hypomania versus mania... it is the presence of psychosis or grandiosity or hallucinations (or all three) that makes it mania. Even if you only have a manic episode with psychosis once, and all your other episodes look more like hypomania, the DSM still qualifies that as bipolar I.
Also, it is a dangerous myth (in my opinion) that BPI is more dangerous than BPII. They certainly both present terrible realities. BPI patients are more likely to do destructive things and be hospitalized, but BPII patients spend a lot more time in depression and are generally more of a suicide risk than BPI patients. In my opinion both are equally dangerous, just for different reasons.