i'm not sure on what size tank's otto's need but i know that otto's work with betta, i was reccommened that but the tank i have my betta in is only a 4 gallon so it wasn't big enough for otto's.
someone will help you out better than me with information on the otto's i've only kept them once in a community tank.
that sounds fine to me...ive had a few ottos before now otocinclus vittatus i think about 4cm max...mine didnt even reach that.
one thing though..its a common misconception that they can just live on algae (well they can if you allow it to build up) but in a betta tank that shouldnt really happen with all the water changes etc, just remember to supplement food..other greens and small live foods etc..
Danny
p.s. i havent kept ottos with bettas but if you provide hiding spots there shouldnt be a problem, they love bogwood etc...