hi
i don't know allegro. In altium, the differential pair length can be set by creating a rule. Whenever the rule is violated, it will throw a DRC error.
its good, now i am ok with creating differential pairs. But i have another one question once i assigned differential pair i need to length matching for that so where i need to set minimum and maximum length?
If you goto DRC there is option for assigning "Length" under "High Speed".
Here you can either assign length to individual length or differential pair through "Advanced query".
You can get answers to all your queries if you search in help file or if you can search it google.