It doesn’t mention Wizards either. Indeed, it specifies nothing that you mention at all. Saying ‘some Pseudodragons are willing to serve spell-casters’ does not automatically grant every PC spell-caster access to new powers. As much as anything else they are making mention of links to NPCs not PCs. A pseudodragon is not a creature that is summoned by the Find Familiar spell. You have interpreted those comments in the MM simply to have your cake and eat it.The only class that details the possibility of summoning these types of familiar is the Warlock via it’s Pact of the Chain. It explicitly refers to these creatures - “Your familiar is more cunning than a typical familiar. It’s default form can be a reflection of your patron, with sprites and pseudo dragons tied to the Archfey and imps and quasits tied to the Fiend.”
The whole point of the Pact of the Chain is that you are getting a superior, exclusive familiar.
If you allow Pseudodragons as familiars for other Classes be aware that Find Familiar is a way too low level spell for bonding with such a potent creature and that you are effectively ruining the Warlock Class as a concept. They don’t have an extensive list of spells like other casters, or the exclusive features granted to those Classes. Wizards, Sorcerers don’t have the thematic ties to grant them access to these beings either - having sacrificed nothing to grant that degree of power. I wouldn’t allow it at all in my game.
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