Not many (if any) 13ts that are legal on a 26t. We only move on 32t, in terms of what to look out for, obviously a 13t is a lot bigger than an 8t when loading, you will likely be slightly overhanging both sides of the truck. I have loaded and unloaded a fair few, often do it for drivers if they are nervous or the weathers rough etc.
If your machine is say a foot wider than the truck bed, just keep an eye on one side whilst going up, make sure you keep it straight and only the 6" overhanging, the other side will look after itself. A lot easier to concentrate that way.
With regard to buckets and a blade, I guess you mean not being able to pinch the spare buckets between the grader and the blade when loading? There's no right and wrong that I am aware of, most drivers have their own way. If you have a bulk bucket with teeth going out have that on the machine and pick the others up with the teeth through the pins, however some companies might not like that. Worth having a 4 or 5 foot drop chain and loading them either before or after the machine. Obviously if you have space its easier to load them before, then you're not rocking around all over the place on the back of the truck.
Bear in mind a 13t is a lot on a rigid, its a lot more unforgiving if you don't put your ramp props down properly you'll bend them, if you dont drop the air out of the suspension its going to be very wobbly! If you are struggling to climb the ramps in the wet with steel tracks, either back on and push yourself with the bucket, or put a chain with plenty of slack in it from side to side of the bed behind the cab and pull yourself up with that.
I have loaded a fair bit of kit on lorries of all sizes, from remote control crushers on artics to track dumpers up ramps onto a tar body 18t, a 13t is a long way up in the air on a 32t.
Take your time and keep the revs down. Theres a few clips out there of people slewing too fast or jerkily and throwing themselves off the side. if you think it might be going wrong, stop, take it back off and start again.