I don't usually mix my own potting mix, because it's more convenient to buy it, but it's a good idea if you already have the extra coir and peat. The bale of Pro-mix BX sitting in my garage consists of 80% peat mixed with perlite, vermiculite, lime, nutrients, and micchorizal fungi.

Both coir and peat hold moisture, so I would add about 1/4 perlite as well as some dolomite lime, at the minimum. This should work great for seedlings.

Thanks! I'll have to take a trip out to lowes for the perlite and lime, they'll probably have soil and this whole point will be moot anyway haha!