Different Batteries in Series

I recently built an e-board but out of my own fault I forgot to check how much has been discharged from the two batteries I used to build my e-board, which were ZIPPY Compact 5S 25C 18.5V 5800mAh. One of them died out and I tried the little NiCd trick to recharge it but not too much since the risk of doing that with recharge is exponential to the voltage and in the end I couldn’t revive the battery. So I decided that I need to buy another one, the only problem is that the specific battery I used is discontinued and pops up as out of stock everywhere I look. So I found a couple of other options from the same company, ZIPPY and I ended up deciding that a 6S 22.V 5.8mAh 25C was the best option because I have the batteries wired in series so there will be no limitations on the capacity and the voltages just add up. The only problem is that I don’t know how much the internal resistance of the old battery has increased. I have only used it for one cycle. My main concern is if I opt to buy the new battery will the new battery wear down the old one due to its increased resistance? Because in that case I would end up having to replace both batteries some time in the future. The two batteries, the old and new one, have the same discharge rates, 25C constant and 50C burst. Any thoughts?