Compatability of Template Across Operating Systems

If I create a Word template with form fields for use by clients how can I
assure that it will be able to be used across different operating systems and
versions including Mac? Would I have to create a separate template for Mac
users?

Would it be better if my template contains a lot of numbers and calculations
to make the template in Excel instead? Again, compatability?

Or, should I do something totally different?

Thanks.
--
S. Ross