When I was first starting to ride somewhere; beyond just learning to control the thing, I remember having sore quads near the knees. Perfectly normal for a beginner, depending how beginner you are. When asking for medical-type advice, please include as much information as possible, including your age, how long you’ve been riding, wheel & crank size, basic seat height.
I’m sure tons of people do on 20" wheels. Not that I ever considered 20" wheels and MUni to be a good match, but it seems to work for a lot of people, especially Street & Trials experts. Ever tried 150s on a 20" wheel? Talk about pedals hitting the ground all the time…
FIFY. 150 was not a popular size in the earlier days (back then I was using 140s on my Miyata for MUni). The reason lots of people used 170s back in the day was that we were pushing the limits of riding difficult terrain. The style of riding has changed somewhat, where it’s now acceptable to simply hop over stuff that’s rideable, since we no longer have as much to prove. Also when unicycles with Profile hubs came out, they all came with 170s. What’s changed since then is that people want a little more speed. And with the addition of brakes, you also don’t need all that leverage to protect your knees on the downhills.
150s better for general riding? No. Waay too long. Better for general dirt riding? Sounds pretty good. I run 140s on my 29er for light MUni…
Sounds like your quads are getting stronger, and/or you’ve raised your seat height. 
Hope you’re enjoying the new 24"!