No. None of these companions can be justified in a truly good party, especially SH.
There are zero satisfactory companions for a good-aligned party right now, and I am deeply skeptical we will get any.
Wyll stinks like demon worshipers, when I first meet him I almsot kileld him on sight. Wyll is strange and crazy mage even if he want do good things often he made mroe harm. Shadowhearth probably will die in late game with selfsacrifice if you let her do good things or some one will sacrefice her if you chose evil way for her.
Paladin is not in game yeat because there is no single companion that will work with typical Paladin. You cannot join githiyanki to LG paladin, you cannot have vamprie that drink blood, demon worshipper is also big NO. Gale is unstable mage, so he can be treath to civilans. Shadowheart is worshiper of evil good, but she is doing good things, mabe paladin will try make her convert to good alightment. In the end there is no way to make good alightment party.
I don't really see why a paladin wouldn't be able to ally with some of these characters. Gale and Wyll in particular are characters that most paladins would gladly accept into their parties. Neither are really evil aligned and accepting them into their party wouldn't threaten to break a paladin's oath or conflict with the tenets of most good aligned deities.
Gale is hardly an unstable person. The main reason he's a threat to others is the bomb in his chest which is very clearly something that's out of his control (it's not like he wants to explode). Aside from that he's very clearly shown to be a well meaning person, if not a little arrogant, who demonstrates pretty sound morals that most paladins would agree with. A traditional LG paladin would gladly accept his help. In fact I'd reckon a devotion or redemption paladin (as well as paladin of good aligned gods such as Torm, Tyr, Ilmater, and Lathander in the forgotten realms) would feel obligated to try and help him solve the issue with the netherese destruction orb since doing so would obviously be a good act that would help him as well as countless others around him. In particular, I'd say that the Tenets of honor and compassion in a devotion paladin's oath would apply here and not just allow but motivate devotion paladins to accept him into their party.
Wyll is a warlock with a fiendish patron, but that does not by definition make him a bad person. Warlocks after all can be of any alignment, with a person's morality instead being defined by what they choose to do with their powers (their actions) rather than where they came from. In this case, Wyll used his demon-gifted powers in order to do good, becoming a famous hero who has helped people. While his motivations aren't exactly pure (the man does love his glory), that still doesn't change the fact that he did quite a bit of good. Some paladins (specifically redemption paladins, ancients paladins, and those of gods such as Lathander and Ilmater) might disagree with some of his methods such as torturing goblins in order to extract information, but they'd still be willing to ally with him in order to try and rein him in. Wyll also isn't a demon worshipper. In fact, it's the opposite. He very obviously regrets his decision to form a pact with Mizora and his entire motivation for finding her is to try and sever it.
Shadowheart is a bit of interesting case since she is a cleric of an evil deity, but that by itself doesn't necessarily mean she's a bad person or that a paladin wouldn't ally with her. Redemption paladins and those of a lot of good gods would try to help redeem her, especially after seeing how she acts. While traditional LG paladins would probably be a little wary of her, they'd still probably be able to accept her help with how dire the circumstances are and the fact that her behaviour for the most part tends to be good. Having her in a party wouldn't put them at risk of breaking their oath.
Like the only people a (devotion) paladin would be extremely hesitant to have in their party is Astarion and Lae'zael. For everyone else (and especially Gale and Wyll), you'd need to be a Lawful Stupid "stick in the mud" sort of paladin to not even consider accepting their help in such dire circumstances.