I'm damn busy these days, so sorry for slow answer,but...let me give you few advices, cause I'm dealing all the time with wps's and pqr's for new materials for me:
first of all, and basicly of all, you need to know something about chemistry; take a look at a certificates of materials; they're always must have included chemical composition of materials, and judging by that (chemical composition) you should find by yourself best filler metal for your materials..for example; everyone (don't get me wrong) use CM2-IG for welding 10CrMo9-10 with 10CrMo9-10 so, what I want to tell you..put it like this; you probably (definetaly) have wps's and wpqr's for some materials you already welded (I mean for welding plans)- take a look at chemical composition of materials and of filler metal, compare them, and you'll know what I mean..it is the same for every materials, mostly it's in chemical composition
then; google catalogs of big filler metal productors (Lincoln, Bohler, Tysenn...) - it is easy to google it and download welding guidelines and catalogs of that kind of companies- if they guarantee for some filler metal that IS for certain materials - you can't miss it
and final, make sure if you do pqr (wpar) that your wire or electrode have approval from approval house that will give you that certificate (TUV or something), because if it don't have, you'll make some cost for your company for nothing..
and remember- 15614 standard tells you that if you can prove something, you can do it...so, brutally said, if you use nifil 625 or some other filler metal for your materials and if all results are satisfied in accordance with standard you doing by (NDT reports, bending testings, hardness test, blah blah....)- just do it ;-)