While the obliteration of the Tau xenoscum is, of course, desirable, and must, of course, be included as part of the end result, they are, however, a lesser threat, and lower priority, relative to the forces of Chaos and the Tyranids.
And, one must remember that my Enemy's enemy is my enemy's enemy. The Emperor does not object to xenos killing xenos or xenos killing heretics, or heretics killing xenos.
There is no inherent need to prevent the Tau and Tyranids from killing each other, and in fact, such would be desirable, even something worth encouraging. It would serve to weaken both at no true cost to the Imperium, while allowing Imperial forces to regroup and reconcentrate in accordance with the existing threat priorities, as the Crusade has not truly been able to do since the appearance of Hive Fleet Dagon.
Lord Ebongrave is not mentally well suited for combating an enemy such as the Tau. And he is especially not suited to engage the Tau in a truce/cold war, official or unoffical, while both the Imperium and the Tau deal with the greater mutual threat of the Tyranids and Chaos. Such a truce would last for the duration of hostilities against Dagon, and perhaps against the forces of the Stigmartus, if the Tau have engaged them as well, but would not last longer. Indeed, while it is to be expected that the Tau will engage in treachery and resume hostilities against the Imperium after Dagon has been defeated, it is the height of folly to assume that the Imperium cannot and will not respond in kind. It is expected that the Tau may attempt to act against the Imperium once our mutual goals have been achieved, but it is also to be expected that the Inquisition and the Deathwatch will engage in superior treachery and deviousness against the Tau.
Realistically, get rid of Ebongrave - maybe transfer him to another front, maybe assassinate him, whatever - but however you do it, get someone more flexible in charge, so that you can cut a short-term deal with the Tau to focus on the Tyranids (the Tau would rather exclusively shoot the Tyranids than continue fighting the Imperium at the same time). This is vital - the Tau aren't going to signifcantly violate the deal until Dagon is finished, which allows you to pull a lot of ground and naval combat power out of this front, and send it against the Tyranids, and lets you focus on counter-insurgency and rebuilding the faith in the Imperium that Ebongrave has squandered amongst the worlds of this front.
Consider seeding a path through Stigmartus rear-echelon worlds to Tau space with Tyranids advance organisms/genestealers. Consider drawing plans to launch kill team, commando, or assassin raids against Stigmartus forge worlds, especially Samech. Possibly Exterminatus some of them.
The Crusade was doing fine until Dagon poked its nose in.
As far as requesting additional forces, I'd say make a request to the Sororitas Militant Orders, point them in the direction of the Stigmartus/Hadex Anomaly, say "there be heretics". And then fetch popcorn to watch from afar, or fetch marshmallows and the makings of 's'mores.