Normally you'd go the other way and use smaller gauge wads inside the shotcup to raise the shot column height to get a good crimp. If you do this the card helps the pattern also, (more for the shot to cushion against when set-back forces act on it). If, however, you use a card as an over-powder wad, and then put a standard plastic wad above that, the card could become squashed and deformed into the plastic over-powder cup on ignition. I'm assuming you're not just building a long column of multiple cards.

If you're worried about the card diameter not sealing, and it's made of cardboard, you can add a few drops of something to it to make it expand slightly. It's mentioned in Robert Stack's Shotgun Digest, which I can look up if you like. It might not work with fibre though. I've used fibre wads and cardboard wads in my 10 gauge putting them inside both SP10 and BPD10 wads. However I prefer insert wads made of wool from Ballistic Products. It's quite revealing to recover the various wads down-range and study how they're marked. It's also revealing to pattern the various loads and compare the options this way too ! I think the woolen wads won out slightly.

BTW, if the crimp didn't seal and small pellets or buffer came through it when agitated, I used parafin wax used to seal fruit preserving jars. Just a melted drop on the crimp gap sealed it. Also what worked equally well was a cut out 1" square of thin waxed paper pushed on top of the shot column just before crimping.