Skills after merging?

Correct - all skills, learned and not learned, will transfer over.

But SP accrued on the old unit would NOT transfer over. You'll get 150 SP from the merge and that's it. I have like 3000 SP on my current Celica, and I have a better IV one just sitting there because I don't want to lose all that SP...haha

The base unit keeps its skills, while the merged unit passes all extra skills on to the base unit. Effectively, in the way you phrased it, the result will be all skills of both units on the newly merged unit.

I was writing an answer, but it turned out to be false info. I cant delete my comment so dont mind this one.