Is there any way to add the Inbox app to the uc() supported apps list? I use it instead of gmail because it's a thousand times better, but I can't find any go-around to get an unread count for the Inbox app. Thanks!

Unfortunately UC depends on the client to provide a way to read emails and counters and right now this is supported only by GMail and K9 (K9 is still not available in Kustom) all the others are not providing such a functionality so there is no way to add support to UC other than counting the notifications.