From what I remember reading Elongated Man at the least has said he could shapeshift in the comics but I don't think I've ever actually seen a story with him actually do it.
I remember the Justice League Unlimited cartoon similarly touched on that with Ralph complaining "Why's everyone talk about Plastic Man? I once hid myself as a vase for 3 days! Nobody found me!" or something to that amount. So at the very least it hasn't come COMPLETELY from nowhere.
Ironically Plasticman's one "weakness" in the comics is that he usually CAN'T change his colors as well (unlike his son Offspring) as opposed to what Ralph did here in the show.
I'm assuming the other bus meta's aren't actually dead. At least if they are it'd be really anti-climactic. I'm assuming like with Becky they're just extensions of DeVoe for now and that his way of "curing' the world amounts to making everyone LITERALLY him.