Oto's are lovely little fish, but I don't think they should be considered part of a clean up crew as such. Sure they do a great job of helping to prevent algae on glass and plants and hardscape, but they have very specific requirements of their own and require direct care and feeding like any other fish we add to our tanks. They can't just be left to their own devices like shrimp and snails can for the most part.
If you are lucky they will eat algae wafers (I have found the Hikari wafers to be one of the better ones) , but many wont, so you have to find something they will eat. Mine love chunks of cucumber (as do the shrimp), and to a lesser degree chunks of courgette. I know other folks have fed them blanched spinach and other greens too. They also like algae based pastes such as Repashy Soilent Green, or you can make your own similar to my recipe here:
Guide: DIY Shrimp/Snail/Oto Jelly Food . . .
I'd would say no, not really. If you are only just seeing diatoms - though Oto's seem to love chowing down on diatoms - it's a sign that you are only at an early stage of microbial maturation and the forming of the 'aufwuchs' that Oto's more naturally feed on.
Wait a month or so to give them a better start if you can - if you can't wait, then at least hold off until you get back from holiday and introduce them without the rush.
Also bear in mind that they are gregarious shoaling fish (in shoals of thousands in the wild), so plan for at least 6-8 fish, as they will hang around together. They are also quite sensitive fish that often arrive at the LFS in poor condition, so try and pick fish from the LFS that have been in the tanks a while, are a decent size with nice plump bellies.