Taking into account the some limitations of the square-cube law, biology, and aviation, I have tried to make a dragon-like creature and a hominid that participate in a symbiotic relationship. For starters, the dragon is reptilian, and related to varanids (possibly in the varanoidea superfamily). In particular, I researched varanoidea as an evolutionary candidate because of the air-sac like structures seen in varanids and already impressive and diverse evolutionary history (if mosasaurs could take to the sea, with a little hand-waving, perhaps these guys could take to the air.)
I would call the dragons wyvern-like and small, compared to other fictional beasts. For the sake of stats, I would put their general size and weight in the category of quetzalcoatlus (150-250kg mass and a wingspan of around 11m). I would say that convergent evolution would be our friend here, as most characteristics are similar to that of quetzalcoatlus, save for the tail and head anatomy. These varanid-like dragons have an aerodynamic, flattened tail and a squamate-based head. Perhaps the wings could be larger, but I would like to leave a lot of physiological qualities vague and up to interpretation because I suck at math, and I'm not any kind of aviation specialist.
For takeoff, the creatures could probably vault themselves into the air using their wings, and because the largest 'dragon' species live in alpine environments, they could rely on the altitude to take off from peaks over valleys as well.
One crucial detail for the dragons is their ability to lift prey. They generally prey on goat-like animals such as tahr and sheep that are typically less than 45kg. This is the characteristic that I expect would help the case of dragon riders. Only the mature adults would make good mount material.
The riders themselves are smaller than humans with the larger men usually less than 55kg. They are human-like primates that have adapted to high altitude environments, hence why taking a ride on a dragon's back may be alright for them.
TL;DR: Could tiny people ride a quetzalcoatlus thing?
Considering physics and biology (I'll save the issues of taming the 'dragons', culture, saddles, etc. for later), would these tiny people be able to ride these wyvern-like beasts? Could the weight the dragons would have to endure be too great for powered flight?
As far as handwaving goes, a little is welcome if needed as you see fit, but the more scientifically plausible, the better. Of course, if no math could line up and/or this is completely unsalvageable science-wise, please do say so.