Hate to say it, but it depends.

If the plant(s) are ones you think you will need to remove now and then to trim the roots or switch out now and then to give them better lighting or a cold dormancy, then yes plant them in pots. Or for that matter if it is a plant that you suspect will try to run amuck and take over your terr, then again leave it in a pot. Otherwise you can just plant directly in the substrate.

As far as the foam, again it depends. How much water do you expect those plants to get? If most of the moisture will be in the bottom of the tank, and you just plan on giving 'sips' to those in the foam then probably no prob. For that matter, why not put some drainage holes in before the foam hardens so you still get drainage?