I was in a similar position last year when I purchased a 'yatsubusa' elm. I should of been more prepared to use a systemic insecticide.I had lots of advice to chop my tree,but did not,which is really not a problem.It is more important to keep the tree alive.Cause' cutting back becomes very fun in the future.I started pinching the tree and it responded until these extra tiny aphids took control.I ended up getting frustrated and stuck it in the ground in very poor condition.It actually survived a very cold winter ,so I dug it up cut it hard and put in a 13" colander this Spring.My point is that it is actually nice advice about cutting back,not that you have to do it right away,just that you can and if it buds back anything like"yatsubusa" there is no problem.Of course learn by doing,try what you want first and enjoy your tree.I left a few pictures of my elm as purchased last year and what it is doing this year after a round of chopping and the back buds.I want it to get air pruned in the 13" colander cause' now it needs to grow quite a lot before I trim it this season because it needs to be vigorous first.PURCHASED IN BONSAI POT,pictures proceed backwards cause' you cannot drag a photo on an iPad.