I am 1 person shop here, so? I make all decisions + develop therefore my environment is somewhat different then yours.
a) Can you start coding immediately after having read about free-format from a book ?
Absolutely. This is exactly what I did. It is good your people are willing to progress.
I am developing on AS400 (including visual OOP) for 12 years and had very small learning curve. In my opinion RPG free is best yet RPG with new functions to utilize and closest to mainstream OOP coding techniques.
b) Is it advisable to mix free and fixed, what are the implications ?
There are no implications that I know of. When I pick legacy source I do all mods in free. For new development you should develop free only.
c) What training is offered out there on free-format ?
Hard to say in South Africa. Call IBM. Search the web.
d) Are there standards that have been developed on free-format ?
Yes. Standardization and normalization is always good.
When you read one of free format books it is all spelled out there for you, plus your own shop coding standards that you should have.