RCS is an extension of IMS, so at the very least it should work on WiFi in addition to LTE.
Since T-Mobile's network expansion is all-LTE, HSPA support is kind of moot, but I believe the RCS spec includes HSPA/3G but requires carrier equipment support. But HSPA-only areas should be pretty sparse in ~9 months.
If you text someone who isn't on RCS, it will fall back to SMS/MMS. But I assume that large downloads will sit in the network until your recipient gets re-connected via RCS (like when you MMS someone but their phone isn't on, it just gets delivered later).