you could also have the LFS test your water & see what results they get & if they match yours...but clownfish & ARE type of damsel fish & can be just as agressive & territorial as other damsels...
now if your resutls match LFS, than go ahead & add a clown fihs that you do wish to keep & skip the other damsel...test the water a week or so later again & see what results you get...if OK, add you can also add a cleaning crew...cleaning crew should not result in too much of a bioload if tight size & not made up of other fish...