In theory, identical speakers in vertical alignment at all bed layer positions = most seamless sound field and the only way to achieve TRUE "timbre matching." So if you found a horizontal center speaker that you liked and wanted to use 3 of them up front and/or 5-7 all around, why not?
In reality, many people use mismatched centers with great results---the key is getting a competent center (i.e. one that provides effortless dialogue clarity which usually includes adequate detail/dynamics for the 50% of the front sound effects that the center speaker also does). There's no shortage of "matching" horizontal centers that irredeemably suck, no matter how inflated the pricetag or pretty the cabinet.
You can get a sampling of those cases here:

Should your center speaker match the same Brand as your...
Should your center speaker match the same Brand as your Front Left and Front right speakers? The reason I ask is I purchased a speaker package but the center speaker is a little weak and awkardly shaped especially when mounted under the TV. It has some kind of European design which seem to be...
PS. There are a lot of much better price-to-performance options than SVS Ultras available, esp. if your usage is mainly HT/TV/gaming rather than music.