Beginner multi size subwoofer setup question

I find it hard to believe an 18 cant play to 80hz, or that 6.5's cant play to 60hz.

why are you so concerned about playing down to 10hz....cruisin around playing test tones? i would filter em at 25-80 with some eq and call it a day, dont make it more complicated than it has to be....thats my opinion

You don't mix different sizes of subs.

first off unless you listen to pure re-bassed music, literally ZERO normal music out there right now goes down past 28hz.  Second. Mixing seperate sized subs will get you absolutely nowhere. AKA phase cancellation. They produce different waves at different locations and the sound waves fight eachother and most likely cancel eachother out or create massive distortion. Its literally widely known as the dumbest thing you can do in car audio next to cutting a stock head unit harness.   

You can use a dedicated midbass driver up front in the doors FULLY acoustically treated and enclosed turning the door into a proper speaker enclosure with deadening materials such as CLD and MLV and closed cell foam.

Then in the kick panels have some dedicated midrange speakers fire up on axis in the kick panels with kick panel pods. Then you have tweeters up top for imaging. This requires an 8 channel dsp, two 4 channel amps 100 rms or higher per channel, bridge one of the the amp to the midbass, the other ran to the midrange and tweeter. DSP controls all the crossover functions. This will fill in the gap you are missing and pull the bass imaging up front making you feel like the bass is on the dash rather than coming behind you. This is the proper way to do midbass. Your idea with those skars is 100% guaranteed to fail because science and physics plus i've done it before when i was young and dumb too always garbage results.