If algae is an issue and why you feel the need to get a snail and algae eater.
You need to first know what kind of algae and what is causing it-other livestock can often be more problematic-plus, since this is a closed system the tank will never support the nutritional needs of the algae eating livestock and you will need to supplement feed. This can cause other problems-due to Betta being piggies and will try to eat their food and the Betta can overeat-causing other problems.
You should never add other species to the tank as a clean up crew-unless you want that species to start with and able to care for it properly.
Bottom line-its our job as the hobbyist to be the clean up crew.
Some species of algae is normal and expected in a container of water-it can also be a sign of a healthy system, it can help the tank look more natural by softening edges and it provide area and food for microorganism that help the health of the system as well as provide natural grazing for the Betta and other livestock. Not all algae is bad, however, since this is a closed system-even the good algae needs to be manually removed on a regular basis by the hobbyist along with the water changes.
Do you know what kind of algae...can you post a pic....