Task creating with workflows

I have met 2 challenges when trying to automate task creating for projects.
1. I wish I could set the due date (and start date) to a date property.
Now i have to add an extra step to edit task to do this, and that brings me to the next challange.
2. I wish i could somehow controll/choose witch task i am editing.
Case: I set up workflows for craeting 4 dif taskes, but when editing them, all are chaging to the same date. I can not controll with task it is editing of all that is assosiated with the project (CRM object).