I personally do not like stainless steel hardware in iron, unless it is of a high quality and grade.

I will either use Cadium plated grade 8 (which is getting harder to find) or black oxide grade 8.

As for the length, it really depends on what you are bolting through.

The rule of thumb is 1-1/4 times the bolt diameter into the threads. And a minimum of the threads showing out of a nut if through bolted.

I use blue Loktite like it's free, not necessarily to lock the threads, but it seals then up and keeps them from rusting.

Hope this helps.

GT