Rootstock dimension for mango grafting?

I am planning to get some scionwood from my brother in some unspecified time in the future within the subsequent month or so (from the tree mentioned on this different thread), and I’ve two totally different seedlings in my greenhouse that I might probably use as rootstock, however would like to make use of the smaller one, which is the seedling of the tree I will be grafting. This is what it seems to be like now:

I do not know if that seedling (about 7 months previous, stunted from a winter in a cool greenhouse) is just too small for profitable grafting, although. I do know there are some grafting methods (Z graft, for instance) that can be utilized when a rootstock is thinner than the scionwood, however I am unsure if these work nicely with mangos.

My different choice to make use of as rootstock is a two year-old Ataulfo seedling that I’ve planted within the floor within the greenhouse, however I type of wish to develop that one out, although it is a a lot better dimension for matching scionwood. This is what that appears like:

Is there any means to make use of the little seedling with out ready for it to develop extra first, or ought to I keep on with the bigger one as a substitute?

I am sorry if this has been mentioned on right here already, I attempted to make use of the search characteristic and could not discover any on-point discussions.