Acer Aureum Growing too fast?!

Hi all.

My mother bought us an acer aureum last year in a small pot.  It did well over last summer, but this summer, it's pretty exploded with growth. 

Not normally a bad thing, but I have concerns that it has been too rapid (pretty sure I read something about acres that grow too quick being prone to disease etc) and also it's growing an odd shape.

I eventually want it to grow as a kind of 'mushroom' shaped tree, so will prune a few lower branches in the winter.  However, as you can see from the pictures, the branches have shot off in all directions, but in a pointed, spindly fashion rather than dense and bushy.

The other issue is that to the left of it, there is a sambucas (black beauty) plant (another gift in a small pot from my mother) growing extremely rapidly next to it (I prune this right down each winter). The loss (or movement to another part of the garden) isn't an issue with the sambucas (a plant I considered more akin to a weed...but it's somehow won me over!) so do you think (in the winter) I should move the sambucas and shift the acer over so it has more space to breath (and grow)?

I can only think the reason the acer has grown that way is the branches are shooting out for light (taken up a little by the black sambucas)?

Many thanks.