Is a quadruplet also 16th notes?

You can pretty much put any amount of notes in a beat you like (within reason of course). I play the flute and get crazy runs all the time. To put 5 notes on a beat you would generally think a group of three and then a group of two. There are other little ways to think about things, such as with a group of 7 notes you could think the words "golden opportunity", each syllable being a note, to get it to flow correctly. It's not uncommon to see even 11 or 13 notes on a beat at a moderate tempo. It's not confusing when you just break it down! :)