which is the natural way to refer to the "young generation" (ex: now its the "millennial generation") in a derogatory way:
若い連中
若連中 // don't say the い
若者
若者達
若者ら
Maybe say something like:
今の日本の＿＿は前の経済的な習慣、例えば愛社精神とか集団意識とか年功序列など、を尊重するというより、給料しか考えずに適当に転職するわがままやつらなんじゃない。
To be honest, none of the five words/phrases you listed sound derogatory. 「若{わか}い連中{れんちゅう}」 might to a very small extent, but the rest sounds just normal.
The real question, to me at least, is: "Should you really use a derogatory term to refer to a group of millions of people?" Not sure what the answer would be in your language and/or culture, but in Japanese culture, I can guarantee that that would be considered highly abnormal. It is not like using a derogatory term to refer to a group of several that you personally know.
I would stick with 「若い連中」 myself. If, however, you still want to know some derogatory terms for future reference, then we have:
「若造{わかぞう}」
「青二才{あおにさい}」
「（くそ）ガキ」
「ひよっこ」
「鼻{はな}たれ小僧{こぞう}」
Somebody stop me!
use 若者 is even formal and can hear from news. Add 連中 is a derogatory way, like old people hating young lol.I've never heard 若連中 in Japan¯_(ツ)_/¯ so ''若い連中'' is suitable to that conversion.
やつらんじゃない?今の若連中<-- We don't say 「若連中」. It should be 若い連中.