There is a useful thread here on Ottos:
Thread 'Some notes on Otocinclus'
Some notes on Otocinclus
In short the collective wisdom of the thread was:
The don't change their dietary preferences with age.
They do need to be kept in larger groups (1 or 2 together isn't healthy). In the wild they are groups of thousands.
You will need to provide supplemental food after they have exhausted the food in your tank.
If you add food (and they eat it) you could add them now, however, this is a bit self defeating.
The larger the tank the better. Yours is too small
They don't grow significantly. If you see a monster 'Otto' then it's probably another species. They do have a very, very large subspecies, however, they are rare and expensive.
My final point would be a number of other fish are confused with Ottos including the Siamese Flying Fox (Epalzeorhynchos kalopterus / Crossocheilus siamensis). You also have Crossocheilus Oblongus the Siamese Algae Eater. The natural language naming of these three seems a bit confused! These three fish do have different needs to Ottos.
My final final point would be (and this is just a personal view) its less faff and much better to use a clean up crew of snails, shrimp, and the duckweed index to help you manage your tank. Keep Ottos only because you like them and the algae eating is a added bonus.