0.020" isnt much??!!

In my line of business thats a mile

You should be fine though, I think my slotter is about a 1/16th and I have to hammer the t-moulding in but it goes in ok.
If it is too tight you could always off set the cutter 0.010" and then route it from both sides.
The best thing to do would be to wait until you have your t-moulding and then test it on a scrap piece of material.