>>23380687
Saber being the only Rider who mixes and matches Books semi-regularly is justified in-universe by several scenes. Like this scene in Kenzan's Swordsmen Chronicles episode or the duel with Slash that basically explain that Touma's creativity, which is a quality the other swordsmen lack, allows him to use the powers of the Wonder Ride Books to their fullest extent in order to somewhat make up for his lack in swordsmanship skills. There's also Rintaro advising Touma on the risks of using Book combos since they're physically taxing, the consequences of which we see multiple times throghout Q1. And there's also an implied risk in using other Riders' Books, like how Saber's use of Peter Fantasista backfired on him before Blades took it and wound up performing much better than he did.

All of these moments serve to explain why the trained swordsmen usually stick to just using the Books that "belong" to their specific powerset (mostly as a way of powering-up when fights get too tough), while a reckless and curious newbie like Touma feels compelled to try out and exploit all the new powers he gets his hands on.