Adding plants to existing 20G tank?

I have a cycled 20G tank (high) that currently has some Java Fern and
hornwort thriving. Substrate is plain gravel 1 inch deep.

Since I'd like to add rooted plants what would be the best way to
proceed? Do I add flourite to the existing gravel bed to build it up
or can I plant the new plants in the gravel and put laterite balls
near the base of the new plants? I've read of others who keep their
plants in small clay pots in the tanks.

Thanks.

Andy