you can get the better version of these skills to the same level like the startingversion
so if you want decomposition level 3 you need putrefy level 3, same with other skills like that
as for dot, depends on how you want your build, dot only is too weak damage for many players, the usual thing is to attack with neurotoxine, this causes decomposition, due to rubys there comes poison-weakness-effect and then attacking with death-howl and lifetap is stronger
acid is less popular for pvp