Hi and welcome.
Please share your Cossacks when build and painted.

It might help to wait for the book, so you can see the lists, however, it can't hurt to start with starters anyway.
As a force to fight them...
Tatars is indeed an idea, as in 1654–1656 they were allied with the Commonwealth fighting the Cossacks.
Transylvanians were allied with the Cossacks and Sweden, fighting in Poland (as far as I know they didn't reach the Grand Duchy of Lithuania, but could be wrong).
The best force, historically speaking would be the Polish-Lithuanian Commonwealth for the period of 1648–1657.
It can't hurt to buy some Tatars anyway. I guess in some point you will get to the division level and then you can have 1 Tatar regiment as allies in your Cossack army (Tugay Bey!).
I also thought Cossacks fought the Moscovites.
I was looking for some "proof", but can't really find good periods, but I know there are some historians here who can help us out.
I think, 1659-1667 there was a Cossack host fighting with the Polish crown against Moscow (Mykhailo Khanenko, later hetman of Right-bank Ukraine). He also led raids against the Ottomans apparently, so skirmishes against them might be cool too.
In the same period Petro Doroshenko was supporting Moscow and fought Mykhailo Khanenko; so a Cossack civil is cool too (and good way to build up to a division).
Hope it helps!
