You can use the knifes most of the time yeah. Shes actually going to be adding a sub weapon to Elle, long as its not dual wielding keyblades its fine. As for taking light along with the two other elements, well, shes not sure she can really allow it. Not really fair to everyone else. Keyblades aren't supper common in this world in the first place so most users are probably pretty new to it.
Maybe later down the line she'll allow characters to get more abilities when everyone gets more experienced but for now gonna have to stick with what shes got on the sheet.