The difference between the heat load the house will pull and what the emitters (coil, pipe in the slab) are capable of putting out might be quite a bit different. Sizing by the second may or may not be overkill, I have no way of knowing. So I'm guessing with the info we have to work with so far.
On my house I'd do another alpha, but I don't mind turning down the radiant til I could swap pumps if the radiant ran a little low on a bitter night.
I would not tee off for the radiant first in line on the main loop or when you kick it in the first day or two your forced air heat could suffer.
But putting a bigger pump would be the "safe" route if you don't want to worry about it, I just don't like burning extra electricity.
Ask three guys and you'll get three answers on flow rate needed based on what order the loads are pulled off the loop. I ways aim for the most efficient use of water temps and use a large delta t if possible. But a Heatmaster furnace return water is dispersed in the tank well enough and with recirc pump flow really low return water temps aren't an issue. On some the return water hits the firebox and can create a wet spot and rust it out. So my advise is for the brand I've worked with.
Is that too long-winded?