LOL, same exact thing happened to me. I actually got the nipples off the first cylinder, but I put too much pressure on the spare cylinder and my first wrench broke on its second or third nipple.
I soaked a q-tip with kroil and dabbed it into the chambers around the threads, let both extra cylinders sit over night and they came out easy the next day. Was quite a relief. Once removed, I washed and scrubbed the nipple threads and dabbed some more kroil on'em before replacing. Shoved a bunch of kroil soaked q-tips into the cylinder's threads as well, and it got a bunch of black crud out.
If you don't have kroil, I'd think other similar lubes could get the job done as well.
I shot my 1858 Target model for the first time last week and am now a BP enthusiast, by the way. I left the 5.5" barreled 1858 and spare cylinder at home because I didn't want to get them nipples stuck on any more than they were - had to wait for a new nipple wrench in the mail.
After putting about 40 rounds thru the target model, the kroil coated nipples removed quite easily.