How much power does a lord have over landed knights and lesser lords sworn to him? Could a lord force them to accept scutage or change taxes or something like that?

Given that he’s their liege lord, quite a bit, although it’s mediated by the terms of feudal service that have been codified either in writing or in tradition. 

As for scutage…that’s not really something where you force someone to accept it. Scutage was a privilege that the liege lord allowed to their vassals in lieu of military service. However, when you impose scutage and how much you ask for per knight’s fee, would be something that the liege lord could do. 

By contrast, feudal taxes tended to be fixed by writing or tradition – which is a big part of the reason why kings and liege lords turned to “feudal incidents,” fines, and privileges like scutage to raise their incomes in a hurry.