Yes, this is valid, and your logic is correct. However, it is not invincible. They can always be returned to the hand, or they can be banished, unless of course you have monsters or spells/traps to prevent this. There is also another flaw: the monsters can be affected by card affects. Therefore, something like Divine Dragon Knight Felegrand can be use to do this.
Anything that the rulebook doesn't state makes it valid.