I wasn't a fan of the Profiles a number of generations ago, but if you like these speakers, I agree with going with the matching center speaker. I've done the mismatch before, and trust me, it's well worth the match for the front three (of course the best match would be another 918 as center, vertically arrayed on the same plane; even tougher with boutique brands because they rarely sell individual speakers, must buy a pair . . .)
Save up for it. Worth it. If HT is important to you which it sounds like it is. If stereo music 90% of the time, and HT is very secondary ok fine get a cheaper mismatch.
If you choose not to follow the above advice, at least stay with the same brand is often recommended.
Also you can send an email to Focal telling them of your predicament. They may know various center speakers of the past that may have a better chance of matching, that you could possibly keep an eye out for.