So basically you're looking into doing Carrier Grade NAT?
Firstly, the subnet to use for this is 100.64.0.0/10. Its reserved for carrier NAT applications
Secondly, I don't believe in the US, as long as you're not under any kind of special legal encumbrance, you have to ensure customer connections can be linked back to real customers. However, consult a telecommunications lawyer, because I'm not one
If you do want to track it, on mikrotik its a bit messy. There's two main strategies: predictably map real ip/ports to customer ips, or dynamically map and log everything. Afaik RouterOS has no facilities for predictable mapping (automatically - you can do it with rules/scripting but you need handlers for every individual IP, it can't be done dynamically), and many ppl have had issues logging connections (not to mention the storage it chews).
Good luck