My dilemma is having multiple record layouts, and having the receiving PIC figure out which format to use.

Using an array long enough to fit the largest transmission seems like a waste to me. My messages will range from sending a single value, to the entire set (possibly 30 values).

One alternative is to blindly use the longest possible array on reception, even if I only send one value. I have no clue if that opens up a can of worms.

I see that you can add a LENGTH parameter, maybe that can be used (got some testing to do).