base 7 is of no consequence - base 7 arithmetic functions in the universe as does base 10 unless you mix base 7 with base 10 in a single arithmetic equation - you would be adding apples and oranges and expect a single answer however there is no single answer when adding apples and oranges together in any universe
if adding a number in base 10 with a number in base 7 one of the numbers needs to be converted to the same base as the other
In the multiverse, different maths rules may apply.
Are maths our brains describing the world around us?
Or are they something concrete, like the tree falling in the forest?
More than just base 7 for the Venusians with that many digits - I'm certain there are maths that we cannot conceive of.
base 7 is of no consequence - base 7 arithmetic functions in the universe as does base 10 unless you mix base 7 with base 10 in a single arithmetic equation - you would be adding apples and oranges and expect a single answer however there is no single answer when adding apples and oranges together in any universe
if adding a number in base 10 with a number in base 7 one of the numbers needs to be converted to the same base as the other