Setting custom colors for series and values will be added in phpdocx 9.5 for Premium licenses (there's no release date yet). The current version only supports using MS Word color schemes.
About the second question, phpdocx 9.5 will include options to set horizontalAxis and verticalAxis font sizes, bold and other styles, but changing legend styles isn't included. We have send the request to the dev team, but if you open a ticket (https://www.phpdocx.com/support) the dev team can do the needed changes and send the new classes to your e-mail.
You can find a sample of the formatCode option in the samples included in the package (examples/Core/addChart/sample_16.php):
'formatCode' => '#,##0.00',
As this is a custom string field we recommend you to get the format code string you want to use from MS Word. This is generating a chart using MS Word and setting a new format code, you can copy and paste the value of the field to the formatCode option.