All this is taper talk is BS. Yes, the tapers are different, No you shouldn't mix them. However if you do, it isn't the end of the world and NO, they won't fall off the bike killing you. Once it is done you just can go back, So you will always have to use a Shimano BB on your arms now. Just make sure that the bolt doesn't bottom out on the spindle when you tighten the arms.
I used Shimano Arms on a Campy Bottom bracket, It had been on the bike for 20 years when I finally upgraded. Older square taper was little closer than what it is today.
So the point is, leave everything alone, readjust the FD and ride. Ignore everyone telling you how you are going to die for making this mistake. If the arms no longer tighten or do fail at some point, replace them then.