Hi, 

If you buy different modules from odoo app store from different suppliers, there is  chances for compatibility issues, one module might collide with the working of the other module.

Same thing can happen if you buy modules from same author itself. Here if the modules that you needed can be provided by a single supplier, you can check with them and see whether all modules can installed and used at a time, even you can ask for a demo link and test by your own. 

If you plan to get the modules from different authors, both authors don't know what is there inside the code of the author module, so they cannot assure you that it will work compatible unless the any of the author has got both modules in their hand. 

If you have developer, you can buy modules separately and if there is issues you can fix/adjust using the resource.

 Thanks