A question about performance and memory

I am trying to optimize my games as best as I can, especially when trying to target html5 mobile. There are many areas I don't really know how though and what is happening behind the scenes. I remember reading somewhere that if you have an active with a lot of animation frames in it it takes up a lot of memory and performance and that you could be better off to only have that object in the frame when really needed. This has me thinking of a situation I am in now:

I am doing a game where there will always be one hero and one enemy on the screen at all time, I am thinking about if I should do 20 different objects for every type of enemy and just have one of them created at one time. Or if I should just store all the different enemies in one single object and just play the appropriate animation one at a time. The last option would be more easy when it comes to the actual code since I only need to keep track of one object instead of 20, but I am thinking maybe it needs to load in all the 20 character animations at the same time though instead of just having 1 loaded at a time?

I hope i made myself clear what I am trying to explain..

Hi outcast, due to my experience animations only eat up memory when they are actually called. I made this observation due to a tilemaking object I use for tiled map creation.

What about using different animations like "stopped" "walking" "running" VS using a lot of direction in the "stopped" for example?

In my case this made no difference, only the single frames that are actually called in the fastloop (and in my case pasted as backdrops) cause memory usage to change. I´m using both methods. I think it´s a bit like using an invisible object; no actual rendering of the animation frame = no memory for the graphics