Hey there!
I saw that you commented on one of our Rancor threads and I thought I'd check in with you here too!
So just to get you on the right track before you get too far into planning, we do not allow crossover style kits. There's a rule about 80% Mando and 20% "inspired by". As long as we see a Mando first and your inspiration second, you might be alright. Be sure to draw up or mock up your ideas before digging into your materials so that we can keep you CRL (Costume Requirement List) compliant. Remember, we encourage our members to create a character that looks like he or she could be seen on a Star Wars movie set. You want to stick to the Mandalorian look as much as possible for approval!
But as for the crest, yes! You totally can do a crest or fin on top, similar to a clone trooper bucket. There are several folks who do a crest/fin on their buckets. And as for colors, normally the colors aren't much of an issue, but for you it might depend on the style of armor you do and your accessories and soft parts (vest, boots, flight suit, etc). That's only because you're going for the Roman influence. But to play it safe, you'll want to sketch up some ideas and keep us in the loop!
Have you had a chance to read our [img=http://mandalorianmercs.org/forum/index.php?topic=148403.0]http://New Member Resource Thread[/img] on the Rancor Clan boards yet? There should be a good bit of information to help you out.
Looking forward to see what you come up with!