So, as far as I understand it correctly, you and your friends (all 3 or 4 of you) want to join SiK and you would like to lead the DD section of SiK but with no further SiK members joining your squad within SiK. Correct me if I'm wrong.
About CoDEx and the problems I encountered, having both race & DD players within one clan makes it very difficult to keep everything in balance and keep everything under control. If one side is stronger or more popular than the other, the section with fewer people needs to be closed or it will just die out. If the two sides (DD & Race) are equally balanced, both having many active players and even separate sub-leaders, then a split can easily happen, resulting in a new clan and both clans will only have 1 game mode. Or you may have two different squads being under the same clan but totally separated, and there's hardly any contact between the two factions. Obviously, there's no point in doing such a thing.
None of these possibilities has a good outcome. In most cases, one side will ultimately become stronger than the other and one of them will come to an end. An acceptable relationship of this DD - Race mixture would be if all members played both game modes, then the two different sides would meet more regularly and it wouldn't be a huge separation. Unfortunately, this scenario is pretty idealistic, as you've already mentioned it's rare to have someone, who can excel in 2 (or even more) game modes. There are some players but clearly not everybody can do that. Eventually, the vast majority of players only play the game mode they enjoy the most (it's simple as that). Specialization (for example SiK is specialized in racing) has its advantages and drawbacks but it's obviously more lucrative than being good at more things. (You have higher chance of becoming successful if you focus on only one thing.)
Also, you mentioned that "it's important to be good at every gamemode". You seem to be an expert of DD but are you good in Race as well?