Is the goal to remove the bottom 10%, 15% of those on probation or is this simply a process everyone goes through before becoming a full employee?
If the former, then you need measurable, hard criteria, not soft ones you listed. It needs to be tied to what they do, something you can count and measure, and then the bottom 10% gets whacked.
If the latter, then I would suggest you have NO criteria, and by that I mean no special criteria other than the same ones you use for all employees for year-end reviews. They have already passed your hiring criteria to get through the door, notwithstanding the extremely flawed interviewing process. To know whether a candidate is a total right fit is going to be more an art than a science. It's a 'you_know_when_you_know' kind of thing. And in some cases, their performance at task is good but they are simply not a fit in the culture, which sort of degrades their overall performance, right? Can you put a point to that? Maybe, but I suspect your gut will be screaming the answer more than a point will show.
But here is the risk you need to mitigate: your personal biases. These are very hard to control. And the more you think you are unbiased, the more you are. While you might be the final decision maker on this, you may want to use a panel approach, where two or three others also weigh in with their findings. This will help keep your biases checked.
EDIT: As I thought more about this, I think I need to mention the HR Department's perspective on things. You MUST check-in with them to learn what the formal things are you need to accomplish. Likely, you will have to conduct a formal employee review, just like the normal year-end reviews everyone else goes through, and rate the employee. HR will need documentation to support removing an employee. So while this might be a gut decision for you, your normal criteria for reviews will need to be used.