Cayde uses solar and arc. A D1 mission had him commenting on his flux grenades he left at a stash not triggering. In D1 it was stated that nightstalkers were extremely rare. It is likely that all guardians can learn almost any ability regardless of class based off of lore about the drifter. There is also evidence of this with felwinter (warlock) using a shoulder charge and hunters learning blink in D1. However, learning to use even same class subs is likely a very long process that could be compared to mastering a martial art in real life. This would explain why most of the guardians that are seen to use multiple elements have been around a long time. The rarity (near nonexistence) of multi class guardians is likely due to the classes channeling the light in very different ways, making learning other class abilities even more time consuming than different elements. Warlocks for the most part just use concentrated raw overpowering light, relying on mostly unshaped light being turned loose on enemies. Hunters are the opposite, almost every use of the light they have shapes it into a weapon. Titans tend to absorb the light into themselves as more of a powerup, with limited ability to make simple constructs such as barriers, bubbles, and shields. Solar titans seem to have more in common with hunters though, but that matches the exile nature associated with them.