Hello and welcome to TPT.
Anchoring the plants - yes, you are ok to sandwich the plants between rocks. As long as the rhizome (horizontal stem) is not buried, i.e. the water can flow though to it, you will be fine. One way of anchoring the plants is to use those thin slippy hair elastics. Goody brand has them in clear.
Your light might actually be ok for the plants, those plants are low light plants so will just grow slowly. If you don't already have one, get a timer and set it for 6-8 hours. Less to start with and bump up half hour or so per week if needed. If you're getting algae, then bump down. If you wish, you can even split this total time into 2 periods of at least 3 hours each. It takes plants about 3 hours to complete photosynthesis, and some of us like to split the viewing time.
Your Marimo ball would like to get turned occasionally if the betta isn't already doing it.
Beautiful fish, and nice rocks!
So bottom line, suggest observing your plants for about a month before buying a new light. It take about that long for slow growing plants to show good growth in low light set ups. I would expect the anubias to have one or two new leaves during that time.
In case you still need to change the light-
Here's a thread on the mini bow, specifically bulbs.
A 9W CFL would be enough I suspect for this tank, anything higher might just be an algae factory.