The guppies living in there will certainly be better beforehand, yea. Something living and pooping in there really is necessary to boost up the biological ecosystem for a truly mature tank (note: I'm differentiating between "mature" and "cycled". Still make sure you cycle the tank before adding guppies.)
Start off low with the fertilizer and watch your plants is the best advice I can give there for now. Every tank is different, and my fert routine will be different from yours. The key point to remember is algae feeds on whatever you have in excess. Too much light, too much of any one macronutrient, not enough of another, these can all lead to algae. You've got us here at fishlore to help you if and when you run in to issues.
Remember, while otos are "algae eaters" they won't eat hair algae, black beard, or tougher stuff in general. You'll want to keep those at bay.
Also, for what it's worth. Most sources will list otos as strictly herbivores, but I've seen mine chowing on
shrimp pellets and frozen food. The fact that I've seen them regularly taking advantage of protein like this tells me that they're likely benefitting from the micro fauna living IN the algae as much as they are from the algae itself. All the more reason to stress tank maturity.