By what impression we were given from Oda, Shanks is strongest Yonko, tho, I could see Kid underestimating him because he don't have an arm.
Kaido shouldn't be strong as Shanks at least in my opinion.
Law said that SH and HP alliance has at best 30% of chances to take Kaido down, since I do think that Shanks is even stronger than Kaido, Kidd alliance would needed to be around 4 times stronger than SHP + HP. Which will not going to happen. :XD:
I could see them fighting Shanks and I would like to see them against Shanks. I think that Kidd underestimate power of Yonko and I do think that at this point, he would lose to any of them, if he lose to Kaido, BIg Mum or BB, I think they will kill him for sure, but if he go against Shanks, I think that Shanks will let him go.