So I have three forks for my bike - one broken and two perfectly functional. Don't ask about how I ended up with so many forks.

One of the functional forks has an Al steerer tube; the other functional fork has a carbon steerer tube.

I was wondering if I could just buy a compression plug and use it with both forks. Are compression plugs reusable? Is it okay to use them with aluminum steerer tubes?

You should be able to.
Some plugs/inserts work better on certain ID forks than others.

They are certainly reusable but have to be compatible with the steerer's inside diameter. For example a compression plug made for a 1-1/8" steel steerer is too large to fit a 1-1/8" carbon steerer.

Before anyone asks, yes, there are compression plugs made specifically for steel steerers. They are commonly used before the steerer is cut to final length so the plug can be moved up and down as the final length is determined. Once the final cut is made, they are usually replaced with a starnut because it's lighter.