Add them together.
Basically, it gains an extra stat point for every four levels, so two more stat points.
Hitpoints, max for first level of class, then roll or use whatever method you use (we do average, round up).
Calculate saves of an eighth level tetori, then add it to the critters existing saves.
Feats... every odd hit die it gets one... so...
Wight:4HD so...
First level of Monk, get a feat, 3rd level, 5th, 7th
For BAB, use an eighth level monk plus the wight, so +9
Basically, the Wight COUNTS as a 7th level cohort (like you initially said), in D&D 3.5 it would have been called an ECL 7 creature (equivalent character level).
So when all is said and done, it's gonna have 12 hit dice, but count as a fifteenth level character.
You pretty much ignore the fact that it counts as a 7th level cohort for everything except WBL (as per NPC) and keeping track of what level it is in relation to you. So you must be 17+ to be able to have this with Leadership.
If you're giving it class levels, then you JUST give it class levels. Its current HD type doesn't matter - if you give it levels of barbarian, it will get d12 HD just like all other barbarians. BAB, skills, etc., are all based on the class not the race or creature type.
The only small exception is that you also change its ability scores a bit.
See here, halfway down the page.
I didn't bother commenting on hit dice type because monk and wight are both D8, but yeah...
Calculate the class stuff, then stick it on top of the wight.
That's gonna be one scary minion.