If you have Sol, why aren't you using her? The only reason why you wouldnt stick Sol in a team is if you have Andromeda and/or another good healer (Nike is not: shes ok, but not great) And in your case, Sol is way better than either of Rs. As for Hero Choices... You can NEVER go wrong with Andromeda. Mordred - some bosses (like typhoon right now) have debuff cleanse, Arthur is pure dmg and one atk buff, which is not very usefull if you already have a good attacker... and in your case, you have Shiva, who not only packs a good punch, but also has exact same buff as Arthur, so if water is your main element, i dont see why you would want Arthur. I mean 100% uptime on atk buff is nice, but not nessesary. D'art... Farming is good and stuff, but honestly you can just stick his snatch on another soul. Yes, it will become 4 turn c/d instead of 3, but instead you will get a better soul. Sieg is a boss killer. Just that, nothing else. But he does his job better than anyone: if you want boss dead as fast as possible, you will find no better option than Sieg. Joan is a good soul, but imo she is a discount Andromeda. granted, there are situations, where Joan is better (if a boss has extremely nasty raging overdrive and you cant stun him be4 he gets it off) but generaly imo Andromeda is better. However Joan is a good soul to have since her EX skills are GREAT on other souls. Solomon... DO NOT TOUCH WITH A 10-FOOT POLE!!! EVER.
TLDR i would go with Andromeda->Mordred->D'art->Joan->Arthur and then we should get new souls: Shingen, Morgan, Hercules, and only after them, if you have spare Soul P layin around you might as well get Solomon.