You can use Lumen and Eloquent in a Firebird database
which is already used by another application, without the ORM making changes
in the database structure?
Yes, it is possible. Eloquent does not change the table structure of the source database (except in cases where you create a migration ). You only need to adapt the Model, such as the primary key field, table name, access to the correct fields, and so on.
The Eloquent Documentation shows you how to do this.
For Firebird support with Lumen or Laravel, you can use this package :
link
Another detail: there is the possibility of accessing only about 10% of the tables
without the risk of alteration, creating only the classes Models
required?
Only the Models you implement will be able to manipulate the tables. What you do not implement if you do not need it will stay out.
Would you like to indicate some article, story or book that would help me
for that?
You can choose to read the Lumen, Laravel, or package documentation I've recommended for this. For you that are starting I do not know if it is a good idea to go straight through Lumen, because you will have to do a lot in the hand.
A step by step to what you want to do you will hardly find, but nothing prevents you from breaking your head and posting something that might help someone in the future:)