A one country pass is only valid in 1 country. For travel to that country, you'll need separate tickets from/to the border tariff point (or the first station in that country is that's cheaper).

For Poland, you'd better check if the pass is really a better deal than normal tickets and whether all trains you're planning to take are included. Not all operators in Poland accept Interrail.

1.if you plan to use that B-W Express for in/out-then you have to pay the normal full-flex fare till borderbridge=cost an awful lot of money, maybe eve more as the advance sparpreis till Poznan or so

2. for In PL on PKP all seats are-at least theoretically, RES-easy to do at counter for 1 PLN=22 cts. BUT at peak times often full, as trains tend to be quite short. For local trains pass ONLY valid on POlRegio and KD-but by now there are some 8-9 other local companies.
3. Fares IN PL are quite low for germans and there are also lots of even lower priced special offers-by far the better option

You can still use a BahnCard for the ticket to the border. Or buy a Sparpreis to Rzepin.