Pokkén Tournament is compatible with all amiibo, according to Nintendo's amiibo page, and not just Pokémon-related ones.
First-run copies of the Wii U fighting game ship with its own take on the collectibles — a special Shadow Mewtwo amiibo card — which allow players to use special characters in the arena. Other amiibo will unlock in-game items, the Nintendo website states.
What those items will be are not yet known. When we played Pokkén Tournament in its original arcade iteration, the game didn't offer much in the way of item options; this, along with amiibo functionality, marks another difference between the Wii U and arcade versions.
Nintendo confirmed the game would feature amiibo support during its November Nintendo Direct presentation. The game's own website includes a section detailing how amiibo work — although it's currently not accessible.
Pokkén Tournament launches March 18 worldwide. Expect more details regarding its amiibo usage — and ever expanding roster — in the lead-up to the spring release.