Mixing front and center

They are both nominal 4 ohms and both sensitivity 86 db, I wouldn’t pick such inefficient speakers myself, but starting with your FL and FR, on paper (I never heard them) that’s a very good center choice,

as it is very important to have similar responses at all volume levels, sometimes watching without sub-titles, and sometimes late at night keeping the volume lower and using sub-titles.

I had a small Bose center for many years, bought a cool looking but too efficient Klipsch center, just replaced it with a moderately efficient Jamo Center.

The Klipsch was too efficient for my dbx soundfield 100 mains, the Jamo much closer to their 90 db sensitivity, and I like the real cherry wood. I adjusted individual speakers within the AVR, but my recall was that the Bose blended better.

You need a center channel for content that automatically switches to 5.1 content, or you need to force 2 channel mode, which I often try, and find might sound better to me than some pseudo surround that somehow got involved. 

No mix front and center. 85 percent of dialog comes from the center... a difference is sonic character is likely to be obvious. Rule number one is same house sound for the fronts and center. Less critical for the sides. 

I would focus on matching tone/timbre, and since your speakers have a silk dome tweeter I’d try to stay with something similar and for that reason would shy away from KEFs with their metal tweeters.  This Aperion gets solid reviews from users and in reviews, they offer a 30-day in-home trial period, which is really nice, and it’s over $200 cheaper than the KEF.  Importantly it has a dedicated midrange driver mounted below the tweeter that is superior to the more common MTM design of most centers in this price range (concentric designs as in the KEF are also good).  Hope this helps, and best of luck.

https://www.aperionaudio.com/products/verus-v6c-center-speaker

Well, first, do you need a center?  If you sit in 1 location all the time, the center is not important.  Yes, I’ll get flack for saying this but I have a lot of professional and personal experience.  The center really shines when you are moving off center.  It keeps the dialogue and action placed where it should be as you move around the room.  

And then yes, matching the timber of your current speakers helps a great deal, but  you can somewhat mitigate this with excellent room correction. 

Above all, avoid 2-way, center speakers.  They have limited off-axis response.   This doesn't apply to concentrics.  You could use a single Dynaudio speaker as well, if you can place it normally.  A normal bookshelf 2-way speakers on their side have lopsided dispersion.