A single call to Serial.read() can only return one byte? So Serial.available() must be used as the condition for a while loop with Serial.read() to put the string back together?
Yes and yes.
This line is probably superfluous.
No, it isn't. But, are your servos capable of turning to 360 degrees? To negative angles? I don't think your limit checks are valid.
The code I was questioning, though, is not the first line of that block. It is the last two lines. Casting the byte read from the serial port to a float is completely unnecessary. Why are you dividing by 100?
In any case, if the monkey is sending relative values, you need to add the relative value to some previous value to get an absolute value. Servo.write() takes an absolute value. Servo.write(5) does NOT cause the servo to rotate 5 more degrees. It causes it to rotate TO 5 degrees.