You do what you have to do!
In the world of speaker design, things get more complicated with 2-way horizontal centers--the classic MTM center (mid-tweeter-mid) when it comes down to beaming/lobing and assorted nastiness. In short, it is all about the center-to-center distance of the outside mids in reference to the crossover point. Basically, to minimize lobing, you want the crossover point to the tweeter to be a low as possible and the midwoofers to be as close as possible to each other.
Looking at the Klipsch, they cross the tweeter over at 1,500 Hz which is (generally) an octave lower than typical 2-ways which usually crossover at 3,000 Hz--this is a GOOD thing. Those 4 inch midwoofers are also quite small so that brings them in closer together....another good thing. The length of a complete soundwave at 1,500 Hz is 9 inches (22.9 cm) and the Klipsch seem to have that distance as the center-to-center distance between the midwoofers.
Going back to the rules, the optimal center-to-center distance between midwoofers would be 1/4 of the distance at the crossover point or around 2.25 inches (5.6 cm) which won't happen. Since the math don't stack up, then you go with real world use--does it matter? Well, yes and no--as the drivers get farther apart it will create lobes and such nastiness but only minor issues. In reality, for most people as long as it don't go past one full wavelength (for 1,500 Hz that is 9.04 inches or 22.96 cm) you should be in there.
I know this is AVS and it is preached that audio should be the number one focus of your life, your room and your reality--but the fun of it is to see how far you can let reality and life in without seriously degrading sound quality. I've learned that when you have people living with you, they tend to not think a room full of boxes is a good thing--far from it.
Basically, you want your system to look like a soundbar but have far better performance and adjustability, that is a good thing but rules still apply.
Klipsch also makes a center with four 4" midwoofers and the same tweeter/horn combo crossed at 1,500 Hz. It is "2.5 way" or the outer midwoofers get rolled off at 500 Hz to prevent lobing and let the inner two 4" drivers operate exclusively as midranges. This gives more surface area for the bass frequencies without messing up the mids--good design. Would the 2.5 way horizontal center be a better design than the 2 way? Yes, with twice the surface area the bass region would have lower distortion and higher efficiency--the point of the design.
Back to reality, if you can--and width is your friend to seperate the LCR--I'd seriously look at the 2.5 way as your center and the 2 ways as the left/right if you can spare the space. This will make the system wider to give better seperation, will boost center channel clarity and efficiency for the critical center movie vocal tracks and match aesthetically the left/right speakers for the win.
The easy answer is to use the same speakers and to not use horizontal MTMs--don't forget to use four subwoofers, add acoustic panels and bolt large speakers for your surrounds in the exact locations Dolby specifies--very easy answer! However, when you can't do it perfectly to spec--that is where the hobby becomes more entertaining and educational. For me, I'm far more impressed to hear great sound in a living room than great sound in a fully treated sound room only area. Along the lines of car analogies, any idiot can buy a 10 second in the quarter mile car off a car lot...just sign the paperwork. I'm far more impressed when I see a 10 second Mazda Miata daily driver that looks normal.
In summation, there are reasons why you don't use horizontal MTM speakers--I'd be the first to bring that up. If it is that design or nothing...or something like a Bose jewel cube--then the Klipsch MTM crossing 4" midwoofers at 1,500 Hz would be a much better design. If you can, run the 2.5 way version as your center to increase clarity and give a wider splay to your left/right. It won't be perfect but NO audio system that is not installed in a custom built sound room with full acoustic treatments would be perfect--and it still won't be perfect! If anyone ever states they have a "no compromise" system--that means they don't understand audio enough to know where the compromises are.
To be upfront, I don't own any Klipsch speakers, have not heard their center channels and don't use MTMs in my system. I do admire them for trying to get the design flaws smoothed out so my advice is "on paper" only. Good luck with your build, may you delight in the sound quality and won't feel the need for endless upgrade cycles. Good luck!