Off grid solar PV system for developing countries
There are quite a few questions here. Lets try to get them in order.


MonoCrystalline PV panels are just a type of panel. They have their own intrinsic properties and for a long time (and still to a certain extent) had/have a better photon efficiency (more current per unit light). PolyCrystalline are just fine as well; they tend to be cheaper to make and buy. As to your project, I don't see a particular reason why you'd NEED one over the other, so either would be fine.
Boost converters are likely not the way to go for laptops. Most require a decent AC line for their power bricks and even if you wanted to charge them with DC (which you can) there are a range of voltage tolerances and not every laptop charges at "19V." There are plenty of manufactures and chargers vary between 19V, 19.5V, and 20V. Charging a laptop at the wrong voltage will damage it and may cause a fire. For your convenience and for the convenience of anyone using your solar station, find an Inverter that can put out a few hundred watts and have two standard wall outlets for whichever nation you're taking this to be used in. This also helps in that you don't have to have 15 different DC barrel jacks laying about to keep track of.
USB 3.1 chargers are 5V @ 900mA. USB charging on older phones could range up to 5V @ 2.1A. USB-C is complicated. Power Delivery standard uses the CC pins to negotiate the Voltage AND the Current.These range from 5V @ 500mA all the way to 20V @ 5A. Quick charge (Qualcomm)is proprietary and so is Dash Charge (OnePlus). You'd likely run into some issues implementing those. Since you asked for a suggestion, 5V @ 2.1A with a USB-A port is the best bet and supports the most phones. Anything other than that is rarer in use and another good reason to have an Inverter for AC lines. If you have those you can have people bring and use their own chargers for their own phones and not run into any issues with charging speed, intellectual property, or other mishaps