Of course you can use saltwater rods for freshwater fishing and vice versa but you may have problems.
The rods used in the salt are normally #7 and upwards.
This is because of the size of the fish and also the size of the flies used.
Saltwater rods normally have a fast action. The are stiff and have a tip action. The rings are usually a little bigger, to accommodate shooting line. Also the reel seat is made of metal and not wood.
Everything must be rust proof as saltwater corrodes badly.
Most of my fishing is in the salt but in summer I fish for atlantic salmon and on smaller rivers I use #8 or #9 Orvis T3 or Zerogravity rods which are my standard rods for bonefishing.
In general I would say that you can use saltwater rods in fresh water with no problem but the other way round is not good as the fittings will rust and a wooden reel seat exposed to saltwater swells up and makes the rod unusable.