we do our pqrs with those materials. although we use a 90/10 argon and co2. we have welded with a trimix that was 90/8/2 argon co2 o2 with great success but it costs too much. we had a very high tolerance for porosity and thanks for that because there was a lot of small inclusions from the 90/10. not enough for rejection but a surprising amount.

http://208.223.216.204/gtsgas/pdfDocuments/pdf%20Literature/ArgoBlendAluminum.pdf
this company does not recommend the mixture you suggested for short circuit.

my self i have had great success with 75/25 as an all around gas for s-6.i am not at all impressed with doing verts up with hardwire without pulse way way to slow and not all of the welders can do them successfully. ive poured a few thousand pounds of s-6 and s-2 ,75/25 down hand with great success.

are the welds all position? back gouged? mig root? more info and some of the very experienced engineers will give you the best advice.
there are a lot of things to consider when choosing a filler metal and gas so the more info you can give the more accurate/informed the responses will be.
hell you may be able to get away with straight co2 depending on the requirements of the weldment.
darren