I've welded a few stainless steel items with c25 using just plain er70s6 wire. Couldn't tell the difference between that and regular steel. It was several stainless hinges I welded onto a mailbox I made. No problem with rust after 3 years.
I understand the reasons against a tank of tri-mix, they are quite valid:
1) cost. A separate tank leased is probably $150 bucks or more by the time you've paid the deposit, paid for the fill, etc. That's a lot of money for something you aren't planning on using more than once every few months.
2) Space. I don't have much space, and if I had another tank, the only place it could go is where recylcing bins are, so where would I put those? There is no space for even one more item in the garage.
3) Need. Non-critical non-food service non-structural occasional items just don't require it. Sure it'd be great to have a separate trimix bottle but hey what do I care it's a mailbox and it's been fine for 3 years without rust on the hinge. So, clearly no pressing need for tri-mix.
Now, there are valid reasons why you would definitely want or need the tri-mix: Food service area, marine applications, or humid locations such as bathrooms/crawl spaces, etc. If you need rust protection on the weld itself, you need the right wire and the right gas. Its just not always needed even though you are working with stainless. Sometimes good enough is better than being out $150 to do it the 'correct' way.
millr210, 125cf tank C-25w/.030 wire. spdgls 9002x. Jet 5x6 bndsw, HF chopsaw, 4.5 and 9 inch grindrs. .