Should I scale a percentage variable?
I think is not necessary. If the variables that are in percentage is between 0 and 1, you don't need scaled them because they are scaled already.
I have a dataframe containing variables of different scales (age, income, days as customer, percentage spent in each kind of product sold (values from 0 to 1), etc). I believe it's necessary to scale these variables for using in a neural network algorithm, for example. 
My question is: The variables that are in percentage, are somehow already scaled, can I apply MinMax in my whole dataset or should I not consider these percentage variables in Min Max scaling and keep them with original values?