I went Iron Warriors. Not alligned with them RP-wise, but my character was just skipping between Legions and thought the Iron Warriors were interesting so tagged along to Arkhona.

I doubt there will be a way for a Chaos player to look like a Fallen except for the Rogue Trade one. The fallen are not that many and they are only getting fewer in numbers as they are constantly being hunted. So if you dont want to break the lore they can't be that common.

Problem with the fallen is that we don't actually know how many there were. Latest fluff from the Horus Heresy novels and rules suggests that Luther was in exile on Caliban for around 10 years (I think), so that's 10 years of recruits that did not make it to the front lines potentially, so for a legion of 100,000 plus you could be looking at a few chapters worth of marines.
As for which chapter in chaos I have joined I figured Black legion to align my self with Cyphers position in the 13th Crusade working under Abbaddon.

I'm fairly certain that GW canon had around half of the Legion became Fallen.

which would explain why the Lion bombed Caliban instead of attacking it directly, numerically he probably knew that he couldn't win that way.

Actually, I take back my earlier comment. After skimming through my copies of the last three Dark Angles codices, I found no mention of the specific number of Fallen Angels; simply that Luther had turned a new generation of recruits against the Imperium. (Thus, the majority of Fallen were Calibanite, not Terran.)

The choice of bombardment was a response to the tactics used by the Fallen. When the Lion's fleet approached Caliban, the Fallen opened fire with Caliban's significant array of orbital defence weaponry, necessitating an orbital bombardment counter-attack. The Lion then lead the assault on the Fortress-Monastery, and the rest, as they say, is history...

So, it's fair to say that your earlier postulation regarding numbers of Fallen is a reasonable assumption. The problem with Heresy era Legion numbers is that it's challenging to reconcile those with present-day Chapter numbers; since there were only three 2nd Founding Chapters from the DA Legion, that only adds up to a mere 4000 marines. Canonically, there are 7 Unforgiven Chapters in total, including the DA; that's 7000 marines. Even taking into account heavy legion losses during the actual Heresy, there are still thousands of Marines. We can only assume that the remaining marines were therefore split into other chapters which have not yet been canonically documented.