Typecasting of promises objects is a must and you must have to convert promises generated from different sources in Q type promises. This is because of the simple fact that not all promise libraries have the same warranties as Q and certainly don't provide all of the same methods.

//using when 
return Q.when(AmIAvalueOrPromise, function (value) {
}, function (error) {
});
//The following are equivalent:
return Q.all([a, b]);
return Q.fcall(function () {
    return [a, b];
})
.all();

Most libraries only provide a partially functional then method. Q, on the other hand, is quite different to others:

return Q($.ajax(...))
.then(function () {
});

If there is any way that the promise you have got is not a Q promise as provided by your library, you should wrap it using a Q function. You can even use Q.invoke(); as shorthand, as shown in the following code:

return Q.invoke($, 'ajax', ...)
.then(function () {
});