Flr m finaciancial requirements

I've to apply flr m visa for my family before 15 aug 2016. The financial requirement is £24800 (wife plus 2 kids).
I'm earning £21000 per annum while my wife earning £8000 per annum.

The problem is my wife's wage was not paid through her bank account (cash in hand).
My first question is
Should i wait for six month until her wage is also reflected in her bank account or a letter from employer confirming har wage as cash in hand should be sufficient.
The second question is
What if i change my job now, (trying to get a better job ), should i have to be with new employer for atleast 6 month to get my income approved as a finacial requirement.