No, 10 gallons isn't too small, but small tanks (less than 50 gallons) have their own set of problems. There are basically two problems that you will encounter with the 10 g that you will not have to deal with in your 55 g. Those problems are water temperature and water evaporation.
Smaller bodies of water do not hold their temperature nearly as well as do larger ones. Therefore, 10 gallons heat up and cool down much more quickly than 55 gallons do. This is stressful to fish. Therefore, you will need to make sure that the 10 g tank is not in any direct sunlight, or near any A/C vents. It needs to be in a room with as stable a temperature as possible as well.
The other issue is the salinity of the water. When saltwater evaporates, it leaves behind any dissolved solids (i.e. salt). This raises the specific gravity of the remaining water. The reason this is more of a problem in smaller tanks is because of the percentages involved. If .5 gallons evaporates in 5 days from a 55 gallon tank, then you have lost .9% of your total water volume, with a corresponding rise in salinity. However, if the same .5 fallons evaporates from the 10 gallon tank, you are looking at a 5% loss of volume, and rise in salinity.
Smallertanks can be and are frequently done in SW. You just need to be careful, and make sure your water parameters stay healthy. Good luck!
