Was it correct for Yehoshua to accept them?
Malbim: No. (a) Even though [according to the Rambam] they were obligated to accept even the Kena'ani nations even after crossing the Yarden, if they accept their Mitzvos, they must accept also taxes and servitude. They did not explicitly accept these. (b) One may not make a Bris with them. Yehoshua was careful, and made the Bris only to keep them alive (not kill them), and the Nesi'im swore about this. They did not stipulate not to force them to work for them.