only pot textures can be compressed if mipmaps are enabled

The way I understand the message- if I've checked the mipmaps checkbox & my texture is NPOT, it's even worse than without mipmaps because without mimaps at least I'd get a compression and with mipmaps I get no compression at all because they are NPOT. This is the way I understand it.

No matter if the above is true or false, 1 thing is for sure, having only or the majority of sprites to be POT is a performance boost, at least on older devices(android). The thing is that the way the game is designed, it'd be extremely hard to have all or the majority of sprites to be POT.

1) Let's say we've got 500 textures in a small 2d game. 20 of which are backgrounds and the rest- in game stuff. How many should be the normal number for POT sprites included in the total number of 500?

2) How can I make the majority of sprites to be POT, taking into consideration that they have definite size?

3) In which situation should I make sure the sprites are POT and in which it woudn't matter much?

4) How can I maximize the number of POT sprites in my game?