You will need only one SPF record, as there can be several rules in it. Check out this article for help setting up more than just Google records in your SPF record. Your DKIM signature will indicate a subdomain in your domain in order to verify the signature key. Google Apps uses the google._domainkey subdomain when it google._domainkey DKIM for search. I'm not sure what mailgun is using, but the likelihood is that it will not overlap.
Update
After a quick inspection, I found this article on the Mailgun website and would have thought your SPF entry should look like this:
v=spf1 include:mailgun.org include:_spf.google.com ~all
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