We just had our dining room which has horsehair plaster skim coated. We did not prime first as we were unaware we had to do that. When i went to buy the paint, the service man told us that we had to use a special primer before we painted the walls. Is this necessary?
In my opinion, you did not need to concern yourself with the priming the wall before skimming the whole wall. Priming a wall before pointing up (touching up) a wall is typically done so when the wall is sanded, the pointing up is the only thing that gets sanded. The primer will endure a bit of sanding, so care is used on that step. That was for informational purposes, does not apply to what you are doing here. I am going on the premise that the whole wall was skim coated. BUT, if your wall was simply pointed up to cover cracks in the plaster, since it is plaster and another assumption will be made that it has a "white coat" finish, the white coat is already strong enough to withstand a bit of sanding too. At least what it would take to smooth the skim coat of drywall mud.
On to the answer, finally.
You can use a regular primer, or you can use a special primer if you want. Also in my humble opinion there are places primer should be used, painting metal or wood for example. That is critical for adhesion of the final protective coat. For walls it is more important to get the dust off the walls from sanding than using a primer. I heard a long time ago that latex paint was self priming. In my experience, when it comes to new work, I usually apply a primer coat. On existing walls that were already painted I did not prime, just painted the color coat, two if needed to cover, giving thought to the self primer notion. On my remodel of my home, I had a lot of paint from other projects left over and used regular paint that was close to the color of the finish I wanted as a primer, although it was not primer paint.
This was probably more than you wanted to know but I was on the subject anyway....