It is unusual for any 2 cylinders to have tappings in the same place, and depending on the size of the cylinder and the area you live in (really) they can be male or female threads, or even 22mm or 28mm compression fittings.
Also check to see if your existimg cylinder has a coil (some even have 2), immersion heater bosses etc. Most immersion heaters now have a thermostat built in, but some have a separate 'stat.
I would, however, change the tank. Usually by the time you acrually realise you need a new one, the existing one has rotted through entirely.
Dont use flexi's, use copper tube. You can use plastic pushfit, but i wouldnt on visible areas like an airing cupboard. I believe it is good practice to use sweeping bends rather than elbows where needed on the cylinder connections of gravity systems.