I just read the size of your established sveni...geez!! Thats almost twice as big as the other svenis...that would DEFINATELY be a bad idea. If you introduce new pikes, best thing is to have them as close to the same size as possible. So you are making the right choice not introducing more sveni. Otherwise your new sveni would either be lunch by the bigger sveni or the polys. You could of course get the sveni and grow them out in another tank, then introduce them, but that would be tricky. You might just get a seperate pair...that would be good! 
