You can go with a tufted tail with confidence. Forequarters and wings of an eagle, hindquarters of a lion satisfy even the most grumpy rule-lawyer. The only exception is you're allowed to have the long horses' ears from heraldic pictures, instead of avian ear-tufts. (Your choice, but long ears are far more common.)
The griffon-culture files were never approved by the immortals. Don't worry about them. You'll fit in fine if you:
* Keep your griffon "altruistic" per the help files, instead of an obvious jerk. (Flaws are great, just not ones that glaring.)
* Don't attack other griffons, unless you absolutely must; there's a strong taboo.
* Avoid socials that imply you have a human face (no smiling, etc.)
* Don't play overly stupid. Griffons are on average more intelligent than humans.
* Keep your description standard: quadrupedal, about as tall as a horse, with the eagle/lion split discussed above. Either naturalistic or standard fantasy colors are great; stay away from parrot-green.
If you want a backstory, I recommend creating a clan out of thin air in any likely place. You can decide their structure and politics for yourself; it makes for a good background. (Since griffons become adventuring adults at age 3, they don't have much life experience yet.) Best of all, griffons tend to be secretive about their clans, so you can work out the details as you gain experience in SK, instead of declaring everything at once. Most clan-names are descriptive (Red Lake, Stonefeather), but don't let that stop you if you have a neat idea.
You'll find the griffon community very supportive. Don't hesitate to ask for IC help, or ask for OOC guidance. Good luck!