Can I use smaller diamond disks on my larger flat lap?

Of course.

There are just a few things to consider before you try. Make sure to cut a notch out of the edge of the magnet on the disk somewhere before you apply it to the wheel head. This will facilitate removal of the disk from the wheel head. You most likely will not be able to get your fingertip underneath the edge of the disk to remove it, but if you mark where the notch in the magnet is on the disk with a magic marker, then you can use a flat head screwdriver to "pop" the disk off your wheel head.

Another thing to consider is how you are going to center the diamond disk. If you have a center pilot on your machine and a hole in your disk, this is fairly easy. If not, then you should turn on your machine (with nothing on it) and take a magic marker and put the tip at the point on the wheel where you think the edge of the disk will be. With the wheel turning, this should draw you a perfectly round circle to help center your disk within.