Chr1sG
9
Public Ids are text type 
But I suppose you could convert text to number.
I’m not going to recommend any particular method involving rank, because I don’t know the consequences.
I’d simply suggest using a dedicated ‘Order’ field instead.
Note: it is possible to use a formula field to get the relative position of a child entity in a collection:
(Find(
[Project].Task.Sort().Join("#" + Right("000" + [Public Id], 4), ""),
"#" + Right("000" + [Public Id], 4)
) +
4) /
5
See here: