say something like "So long as Tenant is not in default under this lease and is operating as a ___________________. the Landlord will not lease space in the shopping center to a ____________." If it has this language then you could lose the exclusive by being in default. It would be best to try to negotiate a lease amendment to allow you to pay what you can and stay out of default.
Some exclusives don't have the qualifier about the default - it depends on what you negotiated. You need to read the lease closely with your lawyer. And without the default language, your being behind in your rent shouldn't allow the Landlord to breach (assuming it is a breach by the Landlord).
It depends on how they described the protection that you have as to whether the UFC gym is prohibited. If the Lease says the Landlord won't lease space to a "full service health club", then you have the question of fact as to whether a UFC gym is a "full service health".
Restrictions on use are often strictly construed, so if they can argue it is a different use, you could lose.
Again, it really depends on what the exact language of your lease says. You should get a good real estate attorney and read the lease carefully with him/her.