It's not so much about the canard "upsetting" the airflow but that you have to take into consideration the shift in the "wing" area in that the canard will be supplying some lift at various points and attitudes during the flight. This will result in a shift of the overall MAC of the model. That's why I suggested moving the CG ahead a little for the first flights.

A cheap but effective way to get an idea of how it'll react is to make up a small 12 or so in span all sheet balsa glider that lets you put on and take off the canard by sliding it in and out of a slot in the nose. Flat wings for this simple CG location trial test model will be fine. Balance it up using the stock layout and then see how much it alters things with the canard in place. Make it all to 0-0-0 but allow for warm breath warping of the tail and canard. Warp in just a hair of up in the elevators for the first tests so that your model is positively stable. Don't bother with dihedral for this sort of model since it SHOULD stay stable enough for simple hand glides. If it falls off and it's not just a simple wing warp to fix it then maybe cut and reglue the last 2 inches of the wing with about a 1/2 inch of dihedral to make it stable enough for test gliding. Play with some different decalage angles by warping the elevators and canard from flat to slightly "positve". For the rear that's some reflex and for the front some down. See how altering the source of pitch in these small amounts affects the stability. It may offer you some ideas for how to scale the front to rear angles. And if nothing else it'll show you what happens to the "just barely stable" point of the CG. And after it's all over it'll make a swell mobile decoration for over the bench...