You could and it would make for a much less heavy mosaic when finished, but two things are against the plan working.
1. Plastic pots don't break like ceramic ones
2. Plastic doesn't always stick to the cement.
There are many places you can get broken ceramic tiles or pots, some may actually be given away, so getting the pieces shouldn't be too difficult, otherwise, think carefully about the type of cement you use.
Many pots are polythene based and this is notoriously difficult to get tile cement or many other forms of adhesive to stick to and not peel off.