So here are a few questions...

Blockchain lets you send BTC to someone, obviously. It has the option of "any address" vs selecting a specific address from a list.

1. Is it reasonable to assume that if I select a specific address, it will limit what I can send to the sum of all the unspent inputs sent to that specific address?

2. Or is there more magic to this? Like say if I don't have enough on that specific address's own unspent inputs (but do from other addresses), it first sends the necessary unspent inputs from those others to the selected one, and then sends from the selected one to my intended recipient, sending the change to whereever I designate?

3. What would "any address" mean? Does it simply mean use any of the unspent inputs from any of the keypairs I have the private key for? So in theory, the "sender" could be a bunch of different addresses I have the private key for?

MOST importantly...

4. If I am building out a service where I want to receive BTC from my customers, what is best practice for this? Give each of them a unique and unchanging BTC address to make payments to? Does this violate the principle then that BTC addresses are not to be re-used? I am trying to avoid burdening my customers with the need to goto my site each time they want to make a payment to me to request a new BTC address to pay me with. I am also trying to avoid a related burden then of having to store all the keypair information for all the BTC addresses of all my customers.

The ideal scenario would have been that I have ONE BTC address that ALL BTC payments from ALL customers come to. I own the private key for this address. This would require me to look at the sending address of payment to know who sent it. BAD idea. The sending address is not deterministic.

Less ideal scenario is I have ONE BTC address for EACH customer. So Bob has his deposit address with me. Alice has own own. I own the private keys for these. So now, I can infer that if a payment comes into Bob's deposit address, I credit Bob. Etc. I am making no assumptions on Bob's sending address. But Bob's deposit address is now fixed. A BTC address is being re-used.

Least preferable scenario is everytime Bob wants to send me BTC, he can login to my site, as for a new BTC address, and pay to that address. He could technically pay more than once to the same address but the point here is Bob can keep getting new BTC address to pay me with. I have to now remember all of Bob's deposit addresses on the backend.

Thoughts? I am I right about these three scenarios?