1) Yes, they can be combined.
1 Doujutsu + 1 Taijutsu.
You cannot, however, have 2 Taijutsu or 2 doujutsu. One from each section.
2) No, you cannot. That would be in the role-playing side of the game rather than in-game.
3) Yes, you can, though you do not HAVE to be a sound shinobi.
4) There is already a set list of elements available. Dependant on your stats, where you place your dragon point every time you slay orochimaru, you can have the option to use any of them. The elements are as followed:
Ice - Hyouton
Lightning - Raiton
Hand-to-hand - Taijutsu
Illusionary - Genjutsu
Fire - Katon
Earth - Doton
Water - Suiton
Medical - Medical ninjutsu
Wind - Fuuton
Sand - Sunaton [well, stated as 'Suna ninjutsu' here.]
Though having a kekkei genkai allows you the ability to learn exclusive jutsu, you can learn those listed varying on your stats.
5) Yes, you can. You can have either curse seal or seven star tatoo, or even both if you know how to do it, with your kekkei genkai.
6) Yes, you can. You can have all your kekkei genkai and curse seal + seven star tatoo
7) You can combine one doujutsu with one taijutsu. That's all. But, yes, you can make an uncanny combination such as byakugan with Shikotsumyaku (Kaguya bloodline) or sharingan with Shikotsumyaku, etc.
But one doujutsu and one taijutsu, that's it.
8 ) Well, that's 3 questions, first of all.
Secondly, we cannot tell you for one: It varies on the player's fancy of the game, and two: It's a game secret which the rules states against.
It all varies on how you can play.
There are maxed out Hyuugas and Uchihas around the game, I'm not saying go ask them, please don't, I'm saying that as long as you can adapt to the kekkei genkai and find out its gimmicks, you can easily level with them.
And to answer your 11th question, you can get both seals, though you need a specific alignment to have a much higher chance than getting it with the opposing alignment.
Edit:
Hope this helped.
If you need more information, visit the shinobilegends.com wiki:
http://wiki.shinobilegends.com/index.php/Main_Page&
Be sure to read in the FAQ.
Regards,
Johnny.