In PBP all variable types stores integer values only so only the size of the number they can hold changes. The resolution is always the same.
I see no, and don't know of any, issues with comparing a BYTE to WORD or the other way around but again remember that BYTES and WORDS are always unsigned integers so a variable containing 253 will be smaller than a BYTE (or WORD) variable containin the result of 100-101.

/Henrik.