1) You can transfer crypto to one of your hidden crypto accounts through a normal on-chain transaction. Of course, an hidden account (created by setting a passphrase) is a totally different and separate wallet than the original one.

2) Nope, the passphrase to create hidden accounts is not the same as Ledger Live's one, as the latter is just to locally encrypt your data (public addresses) if you use a shared computer. Obviously you could use the same password for both, though it's not recommended.

3) If had to manage several crypto I would pick Ledger Nano X as it's the most powerful Ledger device and the most widely supported across third-party wallets and blockchain apps. If someone want to have bitcoin only, there are some extremely safe bitcoin-only hardware wallets, such as Coldcard.