I had to smile at this one as mine look very similar... right down to the soil. Your packing them in there so as to save space and you will make some excellent grafts with these to be sure. I just got in some more grafting stock myself... I'll leave it at that for the moment.

100 dollars, I think I would have let that one go as well. The same old story, too many cacti scions and not enough stock here. I just tossed around 30 variegated Astrophytum aeroles away as I was out of stock plants and could not wait any longer to graft. Saved what I could and tossed the rest. You're grafting stocks are coming along good though. You've got a nice selection of different stock plants which is definitely important and they all look very healthy. The other grafts you've done are much harder than the ones you will try with these stocks I imagine, but who knows what craziness the grafting bug might inspire in you now that you have a few nice seedling grafts underway.

Cambium layers are definitely easier to line up with Opuntia as your stock. Just remember, the shallower the cut the closer those cambium layers are together. So shallow cuts for seedlings tend to produce the best results unless you plan on grafting multiple seeds on an Opuntia stock which is also fairly easy to do. Its always easier to cut deeper later on than to add tissue back as well. I'm wishing you luck with these grafts, but I don't think you will need it as everything I've seen you post looks healthy and fairly vigorous.

Those Opuntia look healthy enough to graft onto. You want them nice and plump like that before you start grafting.

Grafting to unrooted pieces of cacti definitely has pros and cons.

Cons are that if the cuts are too small and not plump to begin with they can easily dry out before the graft has taken. You will also want to use a cacti that is actively growing for this. You can take some precautions such as increasing humidity, placing them in a plastic bag to hold in more moisture, etc. to help keep them from drying out. Another con if your scions are also small they can be damaged by the coban more easily if too much pressure is applied... especially so if they dry out.

Pros. You can save many aeroles this way that might otherwise be disposed of. This is especially important if you have a grafted piece that has rot showing in it as you can cut away the rot and hopefully salvage the cacti via areole grafting. Another positive is that the stock is small and can remain hidden as your cacti grows larger. The smaller stock won't push your cacti as hard which means growth will be slower, but you also won't have to worry about splitting of your slower growing cacti as much. Its a good way to save those odds and ends that you have laying around if you don't have sufficient large stock as well. Yet another pro, you can really maneuver the entire stock around as you candle your new graft as you are going through the steps of securing it to the stock. Any tiny crevices that you see light coming through can be tightened down. This results in less lifting of the scion and can make for a much better graft down the road. Many people graft with cut cacti using smaller pieces to get that natural look and push that is better than you would have if you left the seedling or small cacti on its own roots. You also have less aeroles from the stock to push out pups from the stock this way which can be more effectively trimmed.

The bigger the stock the faster and more potentially it may make your seedling crack or grow in a way that makes it look unnatural. You can offset that by not watering as much and providing more sun. Some small stocks such as Pereskiopsis are still able to create an unnatural look however... in part because they can push the seedling so hard. Everything is a learning experience and you are basically on your own figuring out your own watering cycles, temps, etc. The bottom line... slow down growth if your seedlings start to burst or crack... i.e. do not water or do not water as much.

Pereskiopsis is a cacti. It can withstand drying out of the soil. With that said.... watering, fertilizing, humidity, heat... are all key and in different proportions depending on each other... so that is a learning process that is individual in nature to some extent. I find that Pereskiopsis can generally withstand overwatering much better than many cacti, but so too can Opuntia compressa. Pereskiopsis can also be pushed much harder with fertilizers organic or otherwise than many other cacti. To be honest, I tend to error on the side of overfeeding sometimes with my Trichocereus and Pereskiopsis both and unless your looking to grow show specimens there is not as much harm in it as there would be for other slower growing cacti or cacti grafted to these as stock plants.