GPU Memory : Impact of Grayscale Textures

My specific cuestion :

¿Will 4 Grayscale textures (let's say Alpha8) use the same amount of Graphic Memory than a single ARGB texture (both of same sizes)?

In other words, ¿Does having an "Alpha 8" Texture save runtime memory or is only to shrink the package size?

Long story:

I'm trying to fit quite a lot of sprites in the smallest space possible. LOTS of the sprites are monochrome so I'm guessing that is better to use them as if they had only the alpha channel, and I colour them through tinting ¿right?

Also (this should be like a different problem than fitting sprites) I have some screen effects that use different textures but their grayscale... I end up with one shader that uses like 3 grayscale textures ¿should I fuse this textures using one for each color channel OR Its exactly the same as using 3 different textures of one channel each?

PD: Sorry for the bad grammer/spelling