Not a problem.
Be VERY careful believing what the people at many LFS tell you unless you've corroborated the information elsewhere...often the clerks are well meaning but not overly educated, very young or with very little actual experience, due to the fact that many LFS pay minimum wage.
Another problem is that SOME LFS have employees paid on commission, meaning that they'll tell whatever they have to to make as large a sale as possible.
As to "cooked" shrimp, boiled shrimp os OK, particularly if treated with Zoe, Selcon or such....most fish will readliy accept it and it's actually safer with new hobbyists as the oil has been boiled out, meaning it's harder to pollute a tank through overfeeding.
Fried shrimp are a no-no though!
Mike