You will nearly always find frequency response curves and impedance curves for any reputable driver - either at the manufacturer's site or at the dealer's site (or both).

For more-than-2-way systems, sticking to not only the same manufacturer but the same family or line within that manufacturer will reduce your problems for the woofers and mid-woofers and their so-called "voicing" will be very similar. There is no particular advantage or disadvantage to mixing brands when it comes to tweeters versus the lower-frequency elements.

As a concluding note: yes, you can save money by buying a kit, but you cannot make a set of cheap drivers sound like a set of expensive drivers - paradoxically, that is exactly what some of the speaker companies do through their highly engineered systems.

Made me think of some trivia for the day for those intrigued by the history behind making music. I am fascinated by those ‘somethings’ which sometimes occur beyond a masterful design. The magical almost mystical mixes of serendipitous parts, which can affect sound, like the microorganisms, bacteria and fungal threads in some famous Cremonese wood.

I have always felt strongly that real life is stranger than fiction: For lovers of classical music. Did you know that Joshua Bell now owns "The Red Violin". He sold his 1732 Strad, the ‘Tom Taylor’ to be able to purchase the famous ‘Gibson’ Stradivari with its own convoluted history not unlike that of the fictional 17th century Cremonese violin the movie follows. The almost 300 yr old ‘Gibson’ Strad (described as having very red varnish) was stolen twice in its history and lost for ½ a century was purchased by Bell at a reputed 4 million from Norbert Brainin.