However, there are 7 different USDCoins.
So I used the Help feature and ask which of the 7 should I use?
It would have made sense to tell us about the questions you asked their support. That way we could tell you if their reply makes sense or not. USDC is a token that relies on a blockchain like Ethereum's or Tron's for transactions. To send USDC, you need the underlying network asset as well. So, if your are using the ERC (Ethereum) version of USDC, you will need to have ether to pay for the fees. The same is true for the other blockchains and their tokens. You will have to decide for yourself which of these networks and assets you want to use, and then purchase the needed USDC tokens + the native coins to pay for the transaction fees.
IMO, they are acting like isn't a noncustodial wallet it seems a centralized one.
They are not a decentralized wallet. That support rep that answered OP doesn't know the difference between decentralized and non-custodial. Atomic Wallet is supposedly non-custodial. I am saying supposedly because it can't be verified, and we had those hacking incidents that lack explanation of what happened. But Atomic Wallet isn't decentralized. And since OP wants to purchase their coins with fiat, they will have to undergo KYC as well.
USDC is a custodial financial product. You give your dollars to Circle, they store your dollars and give you USDC tokens instead. Does it make sense to look for a non-custodial wallet for an asset which is custodial in its nature?
It still makes sense using non-custodial wallets even if you are dealing with reversible coins/tokens like USDC. USDC and Circle as issuers can freeze your stablecoins in any address they please. It doesn't matter if the address belongs to a non-custodial wallet or it's from a centralized exchange. They can be frozen regardless. That's danger #1.
Danger #2 exists with custodial wallets where the owners/developers/staff can also take your coins and prevent you from spending them. One good example is in the numerous scams we have seen by Freewallet. So if you are using a custodial service, you can have issues from two different sides. Regular users don't get their USDT/USDC or similar stablecoins frozen by the issuers just like that. If they claim they are, there is something they aren't telling you or they don't know the source of their coins perhaps. Getting into problems due to using custodial services is much more common and easily avoidable.
When you need a wallet that supports more assets, use the Unstoppable wallet.
This is probably the recommendation of many users who have used.
Let's be honest here. The people recommending Unstoppable Wallet do so because it's open-source. That's it. They have done none or very limited testing. I am not saying it's bad, but it's a very weird product that I personally didn't like.