bad answers to NeoCron because it's been a while since i've read it but...
All the True Dragons are the top of the chain for monsters. Both Veldora and Milim are True Dragons, but Veldora is stronger. I think realistically, Veldora might only be weaker than 2 of the other True Dragons since Milim obtained most of her dad's powers.
@Archonchure Veldora is inside Rimuru so he can be freed of the prison/not be lonely. Ifrit is inside Rimuru because he's too dangerous to be allowed to run loose, Rimuru could let Ifrit out at any time he wanted.