Big Mom tried to kill Tama and failed. I don't see her getting in danger as big of a concern since she already took part in a war against two yonkou. You say that babysitting her would be a hindrance to the story, when we've already seen how it actually makes characters like Nami and Ussop braver and more useful by wanting to step up to the fight instead of running away. It's also very easy to give her a buff in strength if needed, thanks to the nature of her ability. She can tame giant creatures in Elbaf, and we are sure to meet whatever was inside that egg Roger used to carry around.
There are very few enemies that would pose a threat to the strawhats right now, hell, an admiral just showed up and a large part of the fandom doesn't see him as a threat. The manga has already shown us that the strongest crews in the world can handle having an apprentice around, those being the Roger Pirates, White Beard Pirates and the Rocks Pirates. By having the strawhats taking on an apprentice after then've beaten the yonkou, it would help to establishing them as on par with them, which is consistent with the phase of the story we're about to enter.
On the other hand, Yamato (a Mythical Zoan Advanced CoC user) joining right after Jimbe, by far the strongest addiction we've had so far, would actually end up lowering the stakes during the final saga. Luffy, Zoro and Sanji just got huge buffs during their fights, we've got an ex-shishibukai and Zeus on top of that, unless we get huge amounts of powercreep during this final stretch it is hard to imagine real threats to the crew if Yamato joins.
It is also possible that both join, then Yamato could serve and the protector role you speak off to Tama, since they both share that Ace connection. But right now I'm siding more with only Tama joining, since she already fits right in, all she's got to do is ask. (If you don't think this is possible, I should remind you that Tama asked to join Ace's crew even thought he promised to fix Wano like Luffy. The parallels are very clearly there) While the only thing Yamato has going for her is that she's constantly asking, and all other things her character is lacking people tend to brush aside as future developments.
There are very few enemies that would pose a threat to the strawhats right now, hell, an admiral just showed up and a large part of the fandom doesn't see him as a threat. The manga has already shown us that the strongest crews in the world can handle having an apprentice around, those being the Roger Pirates, White Beard Pirates and the Rocks Pirates. By having the strawhats taking on an apprentice after then've beaten the yonkou, it would help to establishing them as on par with them, which is consistent with the phase of the story we're about to enter.
On the other hand, Yamato (a Mythical Zoan Advanced CoC user) joining right after Jimbe, by far the strongest addiction we've had so far, would actually end up lowering the stakes during the final saga. Luffy, Zoro and Sanji just got huge buffs during their fights, we've got an ex-shishibukai and Zeus on top of that, unless we get huge amounts of powercreep during this final stretch it is hard to imagine real threats to the crew if Yamato joins.
It is also possible that both join, then Yamato could serve and the protector role you speak off to Tama, since they both share that Ace connection. But right now I'm siding more with only Tama joining, since she already fits right in, all she's got to do is ask. (If you don't think this is possible, I should remind you that Tama asked to join Ace's crew even thought he promised to fix Wano like Luffy. The parallels are very clearly there) While the only thing Yamato has going for her is that she's constantly asking, and all other things her character is lacking people tend to brush aside as future developments.
Won't join, keep coping