Typeless would make sense, but these are negative-energy-linked critters. I guess I'll go along with typeless if you prefer.
Typeless seems the best fit. We didn't give the Ash Quasielemental negative energy damage for its cold attack. I'd be more game to give the Vacuum Quasielemental a negative energy attack, since they "embody the absence of
everything".
I guess Engulf often makes it pretty hard to escape (the gelatinous cube isn't too specific), so I could go along with making it impossible to escape short of killing the quasielemental. But then we need to limit the size of engulfed critters. Maybe to 1 size category smaller?
Going by the SRD Cloaker and Gelatinous Cube, it looks like Engulf is treated as a grapple for purposes of escaping from it.
I'd allow them to engulf anything up to their own size, as the original monster was M-sized and could engulf a M-sized opponent.
Rereading the entry though, it could engulf a
single opponent, so we should probably allow for that in the attack's description.
The following would be a pretty literal interpretation of the attack:
Engulf (Ex) #1: If a dust quasi-elemental rolls a critical hit against an opponent its own size or smaller with its slam attack it can wrap its body around its opponent. The engulfed opponent takes X damage at the end of each round they remain engulfed. Any creature reduced to -10 or fewer hit points by this damage is entirely disintegrated, leaving behind only a trace of fine dust. The engulfed creature is considered grappled and trapped within the quasielemental's body.
However, I think I'd prefer a Save vs Engulfing approach, like an Ooze:
Engulf (Ex) #2: If a dust quasi-elemental hits an opponent its own size or smaller with its slam attack, the opponent must succeed on a DC X Reflex save or be engulfed in the quasi-elemental's body. The engulfed opponent takes X damage at the end of each round they remain engulfed. Any creature reduced to -10 or fewer hit points by this damage is entirely disintegrated, leaving behind only a trace of fine dust. The engulfed creature is considered grappled and trapped within the quasielemental's body.
We could use a grapple check instead, like a Cloaker, but I think the Ooze model feels more appropriate.