If crypto really became a thing, inefficiency aside, we actually managed to implement crypto as a payment method in day to day life at stores etc. Which crypto?
Whichever one(s) they wanted to deal with.
Like would a store be able to just take any and all crypto as payment.
Presuming they set themselves up to accept each one, sure. There's nothing about any particular form of crypto that precludes accepting any other.
If coin A is worth 45000 USD and coin B is worth 0.3 USD is it feasible for a store to accept 0.0001 of one coin or 15 of the other for a cup of coffee? Then expanding that out to all of the different cryptos that exist. Could a store feasibly accept all crypto or at best will there be a handful of viable ones?
Nothing would stop a store from doing so, other than having to track a different price for every item in every kind of crypto. I can't imagine anyone would want to do that, just because of what a PITA it would be.
Would anyone even want that considering accepting one coin leaves you open to wild fluctuations in value, would multiples make it basically impossible to function because price would essentially be random and you would just have to hope that you come out favourably for the majority of transactions.
Congrats, you have successfully pointed out the reasons most cryptocurrencies are not very good at being currencies. It's all the downsides of paying for things with precisely weighed flakes of precious metals, but with a massive energy cost to make sure you aren't trying to give the same silver shavings to two different people.
Now, there are things called "stable coins" that solve some of those problems, which are "stable" because they explicitly peg their price to some fiat currency so that price fluctuations are less of a concern (they're as much a concern as using the fiat currency in question in general).