They're not a "regrade or replacement" - if they were, they'd replace something (along the lines of "regrade legionaries as of penal legion - Reg Bd (I)", or "replace crossbowmen with handgunners - Reg Ps (S)")
Neither, despite the mention of javelinmen, do they have an allowance of the type "X per Y Z", so you can't appeal to that to include them.
All, that said, I'm not entirely sure whether you can have them. The rules on allied contingents are expressed in terms of what troops they can include, and PO are not troops. An allied contingent can only include compulsory troops - does that mean it can never include obstacles or fortifications no matter what, or that you can take them even if they're wholly optional because the restriction doesn't apply? The former interpretation seems more linguistically straightforward but would produce some odd results (no stakes for HYW English allies?).
