Do minecraft 1.8.9 mods work with mods for 1.8.8?

Some do

I suspect some would, but most would not.

Not if you install them in their respective versions. Usually if a mod says it's tailored for a specific version you'd usually follow the instructs of DO NOT MIX.

However the 1.8 line is a special case being that it says it's compatible within its line. If you intend to use mods on 1.8 servers, I suggest using 1.8.0 for a wider selection of mods and stability.