A lot of your questions depend on the size of the BTA. I have 20 of them in my tank and they are different sizes. A small BTA (3" or less) will feed on frozen mysis shrimp. If you try to feed it a piece of silverside or shrimp that is too big, the BTA will attempt to eat it but will end up spitting it out. Keep in mind that BTAs do not have to be fed. However with that in mind, they do much better, grow faster and usually have better coloration when fed weekly. You can feed a BTA just about any meaty food, silversides, shrimp, clams, mussels etc... Usually, I start out by feeding a BTA a piece of silverside or shrimp that is smaller than 1/4". It may look like a very small piece but it is just right for the BTA. Once they are used to eating the silversides, I increase the size slowly. I have had 4" BTAs that would devour a whole silverside 3 times a week. There are just too many variables to give you a specific answer. Try a bit of frozen mysis first and take it from there. Don't try to feed a new BTA more than once a week until it gets established. You usually don't know what it was use to eating before you acquired it.
Hope this helps you....