Porre's flag depicts a simple winged lion, although with a snake instead of a tail.
Wouldn't call that simple, but as I can't remember from the game, is it specifically stated somewhere that the tail is a snake?
You are correct, a Griffin has an eagle head. However, winged lions aren't without real world president. Indeed, there are quite a few present from different areas from the fact, that as you noted, the Winged Lion was a symbol for Mark (and, in turn, the Gospel of Mark). It wasn't just present in Venice, the winged lion can be found on the coat of arms for Lostallo, Switzerland and Province of Grosseto, Italy, to name two. The winged lion motif can be found throughout Europe since Christianity was rather big there for a while. I would suspect that you'd be more likely to find that symbol in every Cathedral in the continent rather than not.
As for if the flag is a Chimera, well the word Chimera can have several meanings. On one hand, any "animal" that is comprised of numerous different animals can be said to be a Chimera (indeed, there is also a human genetic disease that is named after the Chimera in which a single person is essentially their own twin). In that sense, it is used as a classification. Thus, a Griffin could be called a Chimera. In the other sense of the word, it refers to a specific combination that of a Lion, a Goat, and a Snake/Dragon. Of course, in Homer's description it might only have one head (that of a lion), with a body of a goat, and a tail of a snake (ah, but it isn't stated that the snakes head in on the tail, indeed it is just implied that the tail would be a snake's tail). Hesiod, however, does describe the creature as having three heads.
Thus, the creature on the flag could be a chimera, but from its appearance is not definitively so. The Compendium's encyclopedia claims it is definitively a chimera, but I didn't find supporting evidence of this.
If we are going from first impressions, that certainly does not look like a lion's head to me. It is lacking a mane. Indeed, it reminds me very strongly of a head from the old Bernstein Bear story books.
Regardless, unless official literature states one way or the other, it appears to just be a chimerical creature of no specific origin. Mind, it is no less dramatic to suppose the creators removed its main, goat-head, and dragon head than to suppose that the creator removed the Christian symbology.
Your description of Venice as a former world power could also be used to describe almost any area. Take England. It used to have a powerful army and navy and its possessions included several territories and islands, not just the main land mass, kind of like Porre. Britain also has the symbol of a lion. America still has a powerful army and navy (though not as powerful as it used to be), has several territories and islands, kind of like Porre. The same can be said of Spain, Italy, France, Greece, etc. The criteria you are using is that is must have or have had a powerful army and navy, that is must have had numerous territories under its possession at one time, and that one of those territories must have been an archipelago. The Virgin Islands, the Canary Islands, the Federate States of Micronesia, and numerous islands in the Mediterranean could all qualify as this.
It isn't that Porre couldn't be an allusion to Venice, but that your evidence is largely coincidental and could equally apply to many other nations with the same degree of validity.
Perhaps a better question to ask might be, would Masato Kato have known about Venice' history and would he have needed to use that as a source reference to create Porre?
