I'm using a Raspberry Pi zero for a drone. The Pi will be powered by a 11.1V battery. To reduce the voltage and current going into the Pi I'm thinking of using a buck boost converter. If I use this converter, will it be compatible? And for voltage and battery percentage detection purposes, do I also use a voltage testing measurement unit or will the buck boost converter be good enough?
To step down the battery voltage, you need a buck converter. Adding the "boost" feature reduces efficiency, adds cost & complexity with no benefit.
The converter you've found on that website is unfortunately typical of the trash they push on the gullible - it has "disappointment" written all over it. The seller's description is gibberish. At one point, they even admit:
but the voltage and current meter values are not accurate;
Their "Product Description" is a parody on Chinese goods - rife with illiteracy and incoherent claims. Here's how it looks today:
All of that said - the concept of using a dc-dc converter to maintain a regulated voltage from batteries is very sound - in fact, it's the only way to go really. But a sound idea executed poorly is not going to be a positive experience for you. FWIW, there are vendors in the world that are competent and honest. You might find something useful in this collection of buck converters.