@calculatorduck.
Thank you that you are interested in using our service and raising those questions.
1. Yes this could be tracked. We have several transaction types like Income and Trades as described here:
https://cointracking.freshdesk.com/a/solutions/articles/29000018272?lang=en. This article includes of course link to other FAQ articles about Mining.
Poloniex transactions could even be imported via API as lot´s of other exchanges as well.
The gains will be calculated according the method based on purchase pools as described here:
https://cointracking.freshdesk.com/a/solutions/articles/29000031793?lang=en2. You could enter all past transaction and create reports as long as you have subscribed to our service. Actually it is required to enter all past transactions to have consistent reports (see FAQ about purchase pool). E.g. if you signup now you can add data from 2013 (or even earlier) until today.
3. Of course you only pay the difference price once you upgrade to the next level.
4. Sure you can do. The only thing you keep in mind is that a transactions has a unique TradeID assigned by the API provider. This is a "is this transaction already there marker". That means if you delete a transaction which is imported via API it will be re-imported again. If you adjust the amount or other values of your transactions this entry will remain according your changes and not be re-imported.
For specific cases you could always reach out to us via our Ticket tool within your account or sending an E-Mail to
support@cointracking.info