Pickerel water conditions and synos don't match at all... and many synos will be too active, along with my personal experience of them being quiet pushy fish.
However, (now I havn't tried it but in theory...) a raphael cat (striped or speckled though striped would probably be better since they get larger) would most likely be the best tank mate I could think of, unless you get a larger tank. They are armored which means the pickerel probably wont attempt to eat them, and if they do they would quickly be detered, before attempiting to swollow it and thus injure themselves. Raphaels also don't move around a whole lot, so you wouldn't have to worry about it stressing out the pickerel, and since I hope you have a nice planted tank, or heavier decorated tank for the pickerel, the raphael would be hide in the decor.
(I've also found common plecos to be quiet pushy, and have found them on more than one ocassion stressing out my other fish, chewing the slime coat off them, and injuring them, causing death and then eatting it... not a good tankmate for pickerel unless you get one of the oversized lethargic ones...)
