I've been having trouble with using all four fingers and not making them feel stiff or cramped. What exercises could I try to help this? I'm mostly having trouble using my pinky naturally. I was thinking maybe some more complex scales would help.
Yeah doing the major scale up and down would work. Even just doing four frets in a row up and down will get your fingers coordinated. The more stuff you play that challenge you the better coordination and techniques you'll get down.
You'll have plenty of excercises (I never remember how to write that word...), just do them slowly, don't push to hard because you might get hurt.It's basically time and practice, the more you play, the more flexible you'll be. If you play bar chords all your life and all from sudden you start playing chords with voicings and stuff, you're gonna struggle at first until you settle down.
Do it, I study some of those "speed, flexibility" excersices and they're "alright" but I rather study something musical. Play scales, arpeggios, chords. But do it slowly, don't push it to hard from the start. It's a step by step thing. You can also stretch your fingers before and after you play in order to warm up.
Most important thing is thumb placement. With a simple exercise like this:


Try starting with your LH thumb print about halfway up the neck behind fret 2. Spread your fingers out so there is space between each of them and ensure fret with finger tips with each LH finger curling down onto the string - don't allow the 4th finger to straighten/lock out.

When it comes to refining technique, often the simplest exercises are the best as they allow s to focus, observe and correct issues without unnecessary distractions.