One finger, thumb, pick etc., doesn't equate to only one tone. This is easy to prove if you use only one finger etc, and see how many different tones / dynamic sounds you can conjure up.
One big reason for practicing with alternating fingers is it becomes easier to hear the difference and make conscious adjustments. When you become more accustomed to the technique you're practicing you'll find you're freer to chop and change - no matter which finger you use, practically, you can get the same result.
Try and make it as comfortable (ergonomic) as possible so you can avoid injury, down the road. Have a look at the greats to see how they manage things. As has been said here, the angle of your hand can help, along with other adjustments like slight bends in the longer digits. And, as @SanDiegoHarry says, practice.
Do it slow and, after a while, when you're good and ready, speed up - when you get good, crank up the pace. Play at ridiculous speeds so that, when you come back down to reality, it's a walk in the park
One big reason for practicing with alternating fingers is it becomes easier to hear the difference and make conscious adjustments. When you become more accustomed to the technique you're practicing you'll find you're freer to chop and change - no matter which finger you use, practically, you can get the same result.
Try and make it as comfortable (ergonomic) as possible so you can avoid injury, down the road. Have a look at the greats to see how they manage things. As has been said here, the angle of your hand can help, along with other adjustments like slight bends in the longer digits. And, as @SanDiegoHarry says, practice.
Do it slow and, after a while, when you're good and ready, speed up - when you get good, crank up the pace. Play at ridiculous speeds so that, when you come back down to reality, it's a walk in the park