manual shows 8mm to 12mm hex adapters both front and rear, the adapters seem to have different offsets, may want to keep that in mind if you are thinking about trying other 8mm hex wheels, not sure i have seen a set of 1/18 wheels with dissimilar offsets before.
given the over sized nature of the yeti jr tires, couldnt you just use 2.2 or 2.2/3.0 wheels and paddle tires? The OD on these "SC" sized paddles is still smaller than the yeti jr tire.
TowerHobbies.com | Pro-Line Sling Shot SC 2.2"/3.0" Tires F/R Slash (2)
it should actually be pretty easy to find a simple and light 2.2/3.0 wheel with an offset that would work, and bing a 12mm hex takes out a lot of the guess work. though if you were trying to put them on the score and not the RR you would probably need a more traditional 1/18th scale setup.