First, don't be so rigid. It's good to keep the volume of all tracks low enough so as not to clip the master bus when they are all combined. But there is no magic number like -10dB that each instrument should be. If I were combining two snares and a clap, I would mix them at whatever values they sound best at. Maybe one snare's sample is super loud, and I set it to -18.2dB. The other might sound best as dominate, and I could set it to -7.8dB. And the clap maybe is just an accent, and I could set it at -11.5dB. These numbers are all made up, but you get the picture: there are no numbers and there is no formula that leads to good sound. So don't concern yourself with numbers and instead just make it sound good!
Second, whether you group the kick and sub-kick depends on what you want to accomplish. If you want the volumes of each to stay connected as you increase or decrease the volume on them all, tossing them into an edit group is the easiest way. But if you want to process them together, then you need to combine them into an auxiliary bus.
If your goal is controlling your low end, I'd recommend EQing the lows out of every instrument that isn't the bass or kick. Then EQ space for the bass to live by cutting frequencies of the kick, and EQ space for the kick to live by cutting frequencies of the bass. If you want the two to take turns being dominant within any given measure, it's best to side-chain compress the bass to the rhythm of the kick.
And if you're concerned about taming the general low frequency level throughout the course of the song, a multi-band compressor can be your friend (either on a low frequency auxiliary bus or the master, with or without compressing higher frequency bands, according to what your song needs).
