My application should be using 3 types of sign up/in: simple form, linkedin, facebook.
My Entity is implementing UserInterface, \Serializable which means I am not using FOSUserBundle, because my registration is customized by my needs. I have already finished sign up/in forms and is working perfectly according my needs.
When I am googling the isue about authentication via linkedin and facebook in Symfony, I get bundles that uses FOSUserBundle (i.e. HWIOAuthBundle). Should I start creating linkedin and facebook authentication using FOSUserBundle and keep form authentication without, or is there a possibility to not use FOSUserBundle?
Thank you in advance!
Use HWIOAuthBundle. It does not require FOSUserBundle. It allows you to also use with FOSUserBundle, which you're not wanting so just ignore that part of the documentation.
I would have added this as a comment, but I don't have enough reputation yet.
I would also encourage you to look at the Guard component of Symfony as of 2.8. Before 2.8 it was supplied as a bundle. Here it is: https://symfony.com/doc/current/cookbook/security/guard-authentication.html#main
I'm not sure it will be helpful to you, but hope one of the above will help you along your journey.