If the VB3.6 hacks use plugins, they will not work in 3.5.x and viceversa.
You will have to manually edit the plugin elements to match the new 3.6 architecture or the 3.5.x one.
The 3.5 ones should work on 3.6 just fine without the executionorder line. I wouldn't think you would HAVE to have that line to make the plug-in work.
You can import 3.5 plugins into 3.6 fine - they get allocated a default execution order of 5.
You cannot import them the other way as a database error is generated. If you manually add the executionorder column to your 3.5 database (plugin table) then you can import 3.6 plugins into 3.5 and they will work fine (as long as the hooks exist of course, many new ones were added to 3.6).
I've come across quite a few 3.5 releases that don't work in 3.6 due to either clashes with new 3.6 functions or simply don't work. Although saying that about 60% or the ones I did try have worked fine.
This isn't anything to do with execution orders though, it's just plain and simply that the code has changed.
The 3.5 ones should work on 3.6 just fine without the executionorder line. I wouldn't think you would HAVE to have that line to make the plug-in work.
maybe you shouldn't have to but..... unfortunately not a single one of my hacks worked on my 3.6 test forum. I had to remove them all, one by one, to get my forum to work.