Okay, well Ive been learning the song Hyena by Rancid (which, btw, if you know, can you help me out with the strumming?) and noticed this problem I have.

When I play powerchords, when I go from a 6th string root note to a 5th string root note and vice versa, I strum a dead note real fast on a upstroke, and then downstroke the powerchord I was changing to. I do it by habit and it kinda doesn't show but you still hear it.

Also, sometimes, my pinky sorta lies down on the string (like a barre chord) and makes me mess up the powerchord.

Any tips?
Focus on your right hand when you're strumming until you get it down. And as for the pinky thing, you just gotta find a comfortable place to have it that won't mess up the sound. Best of luck to ya.
Solve the pinky problem so that u can control it more, obviously just take your time to make it curl and it *might* be cause by thumb positioning

But the added up stroke generally is natural to a lot of people it keeps the song moving and eliminates that small pause from moving from one chord to another
Yeah well I'm a punk guitarist and everything I play I do it, but out of habit, I don't think to do it I just do it lol.
I find it sounds much better to have a small muted upstroke to fill the space between changing chords than to have nothing at all. Practice changing chords quickly and cleanly if this bothers you.

Ideally you want to be playing the upstroke by choice, not by habit.

As for pinky, maybe try to arch it a little more so it mutes the string below rather than accidentally barring it. Make sure it is still comfortable to play though. Check how low you are wearing your guitar too, if you shorten the strap a little and raise the guitar a little higher it might help eliminate that pinky problem.

That's only if you wear it really low though.
Sweet bro, it'll just be a technique thing then.

If I'm playing 2note powerchords, I use my 1st and 4th finger a lot. Although if I'm changing quickly between 6th and 5th string chords, I sometimes use:

- 1st and 3rd fingers with the root on the 6th string, and
- 1st and 4th finger for powerchords with the root on the 5th string.

That helps change between chords really easy as it means I only have to really move my 1st finger to change chord. But I'd only be using 2note powerchords though. For 3 note chords, just focus on changing cleanly and practice mastering that upstroke so it is completely voluntary.

When you fret the chord, do you use the tip or the meat of your 3rd and 4th fingers? (hope that makes sense)

I'd suggest you just fret a chord and see how your fingers sit. If they're too hard up against the strings, arch them a little more. Play round till it feels right. Pinky can help mute the string below but shouldn't be pressed into it unless you intend to barre that way.
Well, my 1st and ring finger are at the tips, and the pinky is kinds not curved as much