Except that isn't in canon. Unless like the Einzbern's you're directly hacking the grail, which was why the third war was so crazy, the Edifelt's dual sorcery trait allowing two sabers, and then the Einzberns. That's the thing - this grail isn't modified like the Apocraphya one - this was pretty much a dirty transplant surgery, not a reworking of it.
So no, you're wrong. And Extra doesn't really count in regards to classes because it's fucking Extra, it's a different system all together. The only classes that we've seen are the primary seven, plus Avenger, and Avenger was only summoned through the Einzberns freaking taking the grail and hacking the fuck out of it.
For that matter, did you even fucking read into shit?
Saber Lancer and Archer don't NEED to be summoned. They're meant to be summoned, by the three primary families, because they are the hax classes. They and Rider all have Magic Resistance, making Caster irrelevant. Assassin will get killed rather easily by any other class in a straight fight, and Presence Concealment doesn't last or work forever. Berserker is a motherfucking trap card, designed to kill people through the sheer power requirements the class needs. Rider? Rider also can't fight straight up, they need their mount to stand a chance. If they get caught without a mount, or since Spear Beats Horseman, by Lancer, they die too.
You can only have one of any class, unless you game the system. This has only happened twice, in the third grail war. And both times, it was someone gaming the system.
TL:DR you don't know your shit so shut the fuck up