For starters you have to consider that this was a high Spanish noble heading to his wedding, presumably with all his side. So say the wedding has four or five hundred guest, half that number might be coming with himwhich could mean one hundred or more men of fighting age. Not such a bad force.
And all though not in the movie, the reasons only the princes were left was because they were the only ones deemed "ransom worthy." Might be that they just left out the executions of the other folks. Sounds brutal, but that is how stuff worked in the 11th century.

The circumstances involving the capturing of the Moors are of no importance. The event is just a way of kicking the story off in a dramatic way and setting up the relationships between the three key characters in the scene -Rodrigo, Garcia Ordonez and Mountamin- for the rest of the film.