So I think I have a short of medium cage and need a long in order to maintain the correct tension!
Your chain doesn’t skip because of a lack of tension, it’s skips because it can’t hold onto the gears. The chain is only in operation between the top (well, top and back a bit) of the front ring and the top of the cassette sprocket – that’s where all the effective/working tension is. The tension created by the derailleur is to take up the slack *under* the gears, the slack (excess chain) required to allow the chain to fit around your both the smallest and the largest gear combinations – the wider the range of gears, the more chain you need, and the longer the derailleur cage needs to be to accommodate it. Derailleur tension stops the chain from flapping around, stops it coming off (laterally) if you backpedal, but doesn’t cause or prevent the chain from jumping vertically off the gears, which is what skipping is. A stuck link, damaged chain, or worn chain/gears are the causes of skipping.