Depending on the size, it's going to take a minute for them to start eating the BBA. They'll start on any other type of algae first, even stuff you don't notice. I have a small case of BBA, but I hadn't noticed it in my java moss. Since I got my SAE's, my java moss has never looked better. Didn't even have a clue it was covered in algae. It's greener than it's ever been.
Also, I've seen my SAE's eating shirmp pellets and some flake food. To "jump start" you could do a 3 day fast for the whole aquarium. On top of that, sure, they eat BBA, but it's not like they'll eat all of it in a couple of days. Give it time, like a month or 6 weeks.
Also, always good to know the scientific name. I went to Albany Aquarium, they sell them under the common name "Siamese Flying Fox". It was like word vomit. Anyway, I started to say that's not what I wanted, but then I saw the scientific name "C. Siamensis" and took 3 home. I've noticed the big chain stores selling "Algae Eater" or "Asian Algae Eater". Who knows what it is, could be C. Siamensis(probably not) could be Gara taeniata.
And, on that note, Albany Aquarium in Albany, CA has them. They aren't cheap, 3 for $15, but it's a good LFS, so I don't mind the price. Point is, if you are in the Bay Area, they have them, and if you aren't, they said they ship.