Many thanks. All of those make perfect sense (and I already added them) except for the Atagis, because the Argens is located in the east. I just re-checked however and found out this: if I understand Ptolemy right, the Illeris, Ruscionis, Atagis, Orobis and Araurus rivers (in this order) have their mouths located along the Mediterranean between the Pyrenees and Sete, which would put them all somewhere in the southwest.
Otherwise, I'm looking for my next additions towards the British Isles, the Iberian Penninsula and the Alpine region... any preferences what to finish first, folks?
OK, in that case, the following should work out:
Illeris: From the location, probably the Teche.
Ruscionis: From the Location, probably the Tet(Probably the origin of Rousillion, of which the Tet forms the northern border (near enough))
And the Vidrus could be the Ijssel, though the Dutch have made that hard to be sure about.