Can we use other fitness brands for our app?

Yes.* In the absence of an express authorization to “use” others’ trademarks in association with your own, you need to be careful to avoid creating the impression that those others brands are sponsoring, approving, certifying, or otherwise qualifying your service / product. You may be able to state generically that your app is compatible with a sampling of wearables, and then affixing a ™ or ® where appropriate and in small print asserting that those marks are IP belonging to their respective owners. How you might accomplish what you propose will depend on exactly what you’d like to say and where and how you’d like to say it. What goes without saying is that you should consult an IP attorney before publishing any packaging or marketing material involving trademarks belonging to other companies.