In the spring of 2012, I attempted to graft a Pear scion onto my apple tree using Winter Banana Apple as an inter stem. (All of he grafting i do is "Whip and Tongue" grafts wrapped in grafting tape. I then coat the wrapped union with grafting wax) The parent branch was a Winesap Apple branch with a grafted Winter Banana Apple scion from a few years before. (I actually have two of these paired branches.) I then attempted to graft two "Kieffer" Pear scions onto my two Winter Banana Apple inter stems. The "Kieffer" Pear scions started to grow, but died a few weeks later. This spring (2014) I attempted to graft this Apple / Pear union again using the same Winter Banana Apple inter stems. I had just enough branch on my Winter Banana branches to try this experiment one more time. This time around, I used "Sunrise Pear" scionwood. The grafting was a screaming success! (I had enough "Sunrise Pear" scionwood to graft two branches onto my apple tree, and two more onto an ornamental pear. All four pear grafts were a success!) I have no idea why this experiment failed in 2012 and was a success in 2014. I do not know if it had to do with the pear variety used, or if it had to do with environmental circumstances. I will re-post in 2015 with an update. There are a few different treads on this subject so I will be re-posting on those threats also. Below are pictures taken on 07/27/2014.

I purchased a three-in-one plum tree from Raintree last year. It has grown nicely and was well labelled. I weighted down the branches with rope and a stone to make the tree more open centered. Looks like it may be budded for fruit next year. I transferred some chip buds ( my first attempt at chip budding) to other plums that I have, so we'll see next spring if that was successful. I have been grafting scions onto existing apple and pear trees for a few years now. My first reaction when these grafts fruited was that flavor must be affected and in decline when a graft. (Mollie's Delicious on Jonathan had no taste; Vista Bella on same tree was o.k.) However, this year has changed that --Honeycrisp is one of four grafts on Hawkeye Delicious and is wonderful. The Grimes Golden on that tree is also great, and expecting great things from Florina in a few weeks. One thing I think I've learned is to not let the tree overbear.

If you have room for full sized trees and the trees are resistant to any soil borne diseases which you may have, such as Crown Rot, then they will grow better on their own roots. ---- Rootstocks are generally used for 3 reasons: Disease resistance, Dwarfing, and Nursing. Example: If you have Crown Rot in the soil and the tree is not resistant, use a resistant rootstock, with the graft union a couple inches above the ground level. Crown Rot can kill an apple tree within 2 years. A common crown rot resistant apple rootstock is Bud 9. Example: If you don't want a 40 ft tall apple tree and would prefer one that only grows 10 ft tall, use a dwarfing rootstock with the graft union a couple inches above the ground level. A common dwarfing apple rootstock is Malling 26. Example: If you have a hard to root apple, but you want it to be grown on its own roots, graft it onto a nursing rootstock that will nurse the tree until it can make roots of its own. Plant the tree with the graft union several inches below the ground level. In a few years, the tree should have made its own roots. A common nursing apple rootstock is Antonovka.

Well since you included the word "seedling" you actually would have no idea what type of bearing the root stock will have. Seedlings in almost all cases are random genetic crosses, unless you went to great extremes to intentionally make a cross to get your root stock. Outside of that I have never heard of a problem grafting different bearing types together.