There are 49 possible combinations when rolling two 7-sided dice.
There are still six ways of making a total of 7, therefore the probability is 6/49.
In each scenario, there are still only six ways of making a total of 7, the only thing that changes is the number of possible combinations.
7-sided > 6/49
8-sided > 6/64
9-sided > 6/81
10-sided > 6/100