Can I use legal first name in financial statements?

Yes. If there is a custom attribute labeled exactly "Legal First Name", and it is populated, the system will use that name for statements.

Contribution Statements will now pull names from the "Legal First Name" attribute if an organization has this attribute and if there is a name in it.   The attribute needs to be named exactly as "Legal First Name". 
This will happen automatically.  If for some reason you have the Legal First Name attribute but do not want this Legal name pulled into the statements, then just rename the "Legal First Name" attribute to something else, run the statements, then put the attribute name back.  
If you choose a Custom name/Address 1 field on the Contributions Options page, that will still over-ride the first name as usual.