You're not really mixing bit-depths here. What it's doing is when you save an image with an alpha with DXT1 compression (thanks sprunghunt for the name, I'd forgotten) it multiplies your alpha into your RGB. Still three 8-bit channels, but containing four channels of information (three 8-bit and one 1-bit).
I don't think you can mix and match the bits-per-channel on an image? Maybe you can... on some kind of weird custom format. It seem more likely a 1 bit alpha on an 8 bit-per-channel image would still be 8 bits, but you're only using a tiny part of it? Kind of like having a 8 gallon gas tank with only 1 gallon in it? The…
This is true if you're using an 8-bit alpha, but if you can somehow find a situation where a 1-bit alpha will work (which granted is almost never useful for a spec map, but would be useful for a glowmap or color variation mask or something of that sort) and you're using dds files, the alpha is essentially free after…