my 3 questions are:
can i filter these so it will look nice in small size, and how (just so i know) without change thier resolution?
when i will change their resolution, how to set billboard so they will be 1:1 pixel of their size?
is dds better to use in animations? how to set dds, so it will have transparency? i tried to convert them to DDS, RGBA with alpha and mipmaps, but never managed to get that transparency to work
If you need alpha I think you need either dxt3/dxt5 as dds (forgot which one).
Quality degrades a bit still, bc7 is much better but encoding time is rather insane (and it will not work in dx9 anyway).
DDS consumes more disk but will be faster to load(no conversion needed, direct upload).
If you need mipmaps, I would still save/load your sequence without mips and generate them afterwards, you trade some (small) amount of gpu power for something like drive/memory bandwidth on upload, which is not negligible.