Tone Change: Why is 欢 (huan1) in 喜欢 (xi3huan0) pronounced with a neutral tone? Would you do that for all words ending in a first tone? Or the 西 (xi1) in 东西 (dong1xi0) is pronounced with no tone?
No. It does not always apply.

xi0 might be another way of writing the neutral tone. I haven't seen it before.

For 东西, dong1xi1 (east and west) and dong1xi(thing) have different meanings.

To make sure whether a word ends with a neutral tone, without hearing how natives are using it, you'll have to look it up in a dictionary.