What host varieties would you suggest and what are the relative carictaristics of each?
What do you do to prepair the seedling it's self for grafting/how is the seedling joined to the host?
I'll think of some other things to add here, just can't think of anything else atm...
I'm not going to to into each and every cacti as a potential stock plant. I will tell you my favorite for growing seedlings fast is Trichocereus. My favorite for growing many seedings a bit slower is Pereskiopsis. If I want something in-between then I choose Quiabentia zehntneri. Its all about the space though. If I graft onto Pereskiopsis... I try to get my seedling off and onto something a bit bigger sooner rather than later. If grafting onto Trichocereus I don't have that concern.
Preparation: If growing hydroponic or bog-ponic seedlings you don't need to worry about a long soak, but I still tend to soak for 15 or so minutes regardless as it knocks off some of the mud clinging to my seedlings. You will also want to cut your seedlings to remove a small bit of tissue so you can position your seedling off center of the stock growth ring/vascular ring. I've presented some pictures of that already.
You may position your seedling anyway that is comfortable for you to take that small bit of tissue off to allow you to place your seedling onto of your stock plants vascular ring. The straighter your seedling stands the better in most cases. Here is one way I have done it in the pictures to follow.
The seedling having been cut in half either horizontally or vertically and placed on the vascular ring will naturally become a part of that other cacti it has been grafted to provided it doesn't float off via the cacti/stock's own juices, get knocked off, or dry out.
You can increase humidity to prevent drying out, or you can simply use parafilm. I use parafilm to prevent drying out as well as to prevent the seedling from floating off or getting knocked off. I can literally graft my seedlings and then throw them across the room and they will still take as they are secured with parafilm.
The bottom line when looking at potential stock is that you ideally want it to be well rooted and actively growing. You want to cut into fresh green growth that is not woody. If you find your stock is even slightly woody, you have cut too low and or used too old of a piece of stock. You are going to evaluate your own stock cacti as you realize how that stock grows and the speed at which it grows will be transferred to your seedling.
Important: If you don't want to use parafilm... do not water your stock cacti a few days before and after you are going to graft. If you are going to use parafilm... water away. If your not using parafilm... keep the humidity high. If your using parafilm don't worry about a thing. If your going to have strong winds or people banging into your grafts... use parafilm.
With parafilm your not going to have to remember to remove anything or do anything special. Just graft and leave it in place. One more important thing though... don't put more than one layer on top of your graft. It has to grow through that thin layer you left in place. It is the sides of the graft down at the stock that will have a thicker layer of parafilm simply because the top piece is touching the layer you wrapped around your stock before you began.