Rey is really quite simple IMHO: the easiest lvl5 mods... health. Whenever anyone encounters Rey, they will drop her first. Fact. She is so utterly devastating (I'd go as far to say game breaking with her foresight and health steal) that any player worth their salt will stack up the health bonuses to ensure she gets at least one turn. After she's had that she gets a lot harder to kill. You don't need to over-empower her damage output - it's already overly powerful. All you have to do is keep her alive. I am currently sat at #1 on my shard and due to inactivty can find myself down to #20 on some days. I have only seen one Rey that hasn't been max lvl5 health modded and they were buffed with speed and health. She died before she got a single swing off.
Leia is a different beast. I would probably train up a Jawa squad for the crit damage challenge as she has naturally high crit chance and then put your last two on anything else you think she's lacking in.
People have the wrong idea about mods. Everyone wants to improve what's already good when what they should be doing is improve what's lacking. I run droids and although I feel a bit short-changed by not having my own challenge, I'm fortunate that the thing I really need is easy to come by: health.
Healers and tanks are usually exceptions to this because they're so specialised. A tank just has to soak up abuse so he'll want extra health to do his job longer. Some healers restore health based on their own health count so they're probably worth maxing out on health bonuses to benefit the whole team.