Actually 20 rep squats are an excellent way to build muscle!
There is an classic book that you should read entitled: “Super Squats”. In that book the author, Randall J. Strossen gives you a complete guide on how to gain plenty of muscle fast using the 20 rep squat system.
At the crux of the program is taking a weight that you normally do 10 repetitions with and force yourself to do 20! While you only have to do one set of 20 repetition squats per week, if you are like me you will have nightmares the night before your leg day. I have gone through the program and believe me, it is tough!
To give credit where credit is due, the first person, to my knowledge, to introduce 20 rep. squats onto the strength/bodybuilding scene was Perry Radar the original publisher of Ironman magazine. He was the owner of that publication when it was a good magazine filled with productive routines. I cannot state the same for this magazine today.
As far as the other exercises that you mentioned. All of them are good, however Squats are the king of leg builders. If done properly, and there are many good routines in which to use, sqauts will build your legs to huge proportions!
Best Of Luck To You,