Your Guitar most definitely has a truss rod in the neck, although it may not be one that is adjustable. If the guitar has no adjustment cover at the head, have a look with an inspection mirror inside the soundhole to double check for an adjustment nut at the neck block. I mention this just in case the music store neglected to look.
I doubt very much that it would be a problem to go to a lighter gauge string as long as you stick with acoustic guitar strings and stay away from electric slinky strings. Going one gauge lighter may cause the neck to change a bit but probably not enough to make a huge difference, and you would enjoy playing it a bit more I'm sure.
Cal
To check the amount of neck relief yourself:
Sight down the guitar neck from the head end towards the body, line up the E6 string with the top of the fingerboard edge. This allows you to see how much neck relief the guitar has because the string will be a perfectly straight line.
Now do the same on the E1st side of the neck.
The neck should have a very small amount of concave(dip) relief when compared to the string. If it has a convex bow(hump) you will probably get string buzz. If it is dead straight it may play OK with some slight buzz depending on the string height.
If the neck rod is not adjustable a good guitar repair person usually can correct a problem, but unless the problem is severe it may be best left alone.