It's time for me to do some housekeeping!

I need some advice on what is safe to pitch over the side. I've been hoarding pretty much everything and buying slots, but that's a bit extreme (and costly -- yeah, I hoard resources, too) so things have gots to go. I just don't know which things and it'll be my luck I'll toss something and regret it later when I get killed by it. So...

- Are there any one-, two-, or three-star characters worth keeping and upgrading, or should I just pitch 'em when the event calls for it?
- Are there any of the more common talismans that I should be keeping? I've been getting rid of the spare ones that seem to farm really easily (Health, for example).
- Should I hang on to leftover fragments from events? Do events repeat at some point, making them useful, or is it a one-and-done kinda thing?
- How long is a good amount of time to keep inactive troopers? I've been holding on to them for seven days but don't know if that's excessive or not (I play every day for about 6 hours, if that matters to things)?

Sorry if some of this has been asked before; I'm still a newbie and some of the threads I saw were a bit outdated.