Finding the right pick for anyone is based largely on personal preferences. However, using a very thin flexible pick less than about .60 mm will result in some degree of snapping off and slapping onto strings, thereby increasing string noise. If that's the case for you, then I'd suggest trying a thicker pick (e.g. 0.73, 0.88) on acoustic and even thicker on electric. String noise from picks is always there, however faint it may be. I think most people eventually tune out these noises, voluntarily or involuntarily. But the softer you play, the more noise you create with your pick relative to the overall sound.

Tonal differences in upstrokes and downstrokes (both picking and strumming) are typically seen as desirable, as they results in a more interesting overall sound. Grasping the pick too loosely will definitely increase string noise. You need to find a pick that you can grasp firmly all the time. I suggest purchasing a guitar pick variety pack to find a pick that suits you.

As far as different strings ringing out disproportionately, that's most likely related to your guitar. Some guitars are intentionally manufactured to emphasize one or more of the tonal zones - bass, middle, treble, rather than being equally balanced across all 6 strings. Many inexpensive guitars are just not structurally capable of a balanced sound.

Nicolai