A quick question:

I somehow lost my Gideon Argus mini. Now I want to replace him with a 'Rex' Rebel Trooper-figure from Star Wars Legion. Would I be allowed to use this figure for Gideon in official tournaments, also considering that it is unambiguous for any opponent who that figure represents , or would it simply not be allowed. Many thanks in advance for any clarification on this.

It depends. At premier events I highly doubt they would allow you to play with a completly different model. Judge has last word, though.

Maybe you can try here: https://parts.asmodeena.com/

Otherwise I’d try to clone it or buy the single model at ebay, etc.

ebay is the cheapest replacement (not too cheap as he is probably the most used figure from the whole core set)

I have a spare Gideon. Just contact me via pm, if you are interested in buying one ...