Hi Rachel,
This is similar to another question asked just yesterday and the same could apply very well to yourself too.
HREFLANG is used in cases like this where a site could encounter duplication issues through this kind of setup. There are two areas to look at and consider here - this answers your question if you decide to keep all TLD's the same.
**Q: <a name="q16"></a>Can I use “rel alternate hreflang” across domains / across TLDs?
A: **Yes! Keep in mind that this markup is on a per-URL basis, so if there are alternate URLs across those domains, you would need to add the markup for each individual URL.
You can get many questions answered here: https://sites.google.com/site/webmasterhelpforum/en/faq-internationalisation#q16
If you decided to pull everyting into just one site, then this would fit what you want to do:
- Your site content is fully translated. For example, you have both German and English versions of each page.
https://support.google.com/webmasters/answer/189077?hl=en
In terms of inter-linking, this seems to be down to the whim of Google, if they wish to see it as problematic or not. If you are concerned about it, leave the links in place and just add the rel=nofollow attribute - this will remove possible issues.
I hope this helps.
-Andy