Is it true to say 51% of nodes should validate a transaction?
No, it is not necessary for any node validate a transaction before it is included in a block and thus added to the blockchain. A transaction can be included in a block without being validated, but doing so risks the transaction being invalid and thus making the block invalid. This has happened several times in the past; miners will make their own transaction and include it in their own block without broadcasting it to the rest of the network.