Basically, you will have a max spoke tension in respec to the rim you are using. I have the Park tension gauge TM-1 and it comes with a set of tables to translate the gauge reading to a tension based on what type of spokes they are. For me personally, I have only used it to equalise tension of spokes. You want to have the tension up towards the higher end of recommended or just under max for heavy riders. With 36 spokes, you could probably even start a little lower. If there are then issues you can up the tension a little, but still staying under max.

I recently killed a Velocity A23 rim and have replaced those wheelset rims with Open Pros as they have the same diameter, thus I could just reuse the spokes. I will see if they can last just as long.

I think you should definitely take the wheels back to the shop and pass judgement on the service you receive. You're a bigger guy, yes, but that is definitely not a reason to break a spoke on a new 36 spoke wheel.

A couple of questions though, did you take the wheel back after a couple of hundred miles to be checked over, and indeed, did they ask you to? Did they build up the wheel themselves? It is a general rule to have a new wheel checked over after a small distance as often the wheel will bed in and go out of true a little. My second wheelset never went out of true until the rear died. The rebuild of that set with the new rims went out of true after only 50km, but has stayed true since. I have since come across talk of newer Mavics rims being difficult to keep true in the rear, so I will keep my fingers crossed.