I have read somewhere on an old post written by users from 2017 that Cloudflare was free to use for enterprises, but didnt found anything related on cloudflare, so to be sure, can Cloudflare domain free plans be used for enterprises?
If you mean whether the free plan is off-limits for companies, then no, you can run a commercial page on a free plan as well. Just make sure you follow the terms of services.
There’s no issue with that as long as you stick to the terms of services, particularly when it comes to what kind of data you transfer (should be mostly HTML, if you use the service mostly for non-HTML content Cloudflare might suspend your account if you exceed certain limits).
Non-HTTP services will have to be unproxied anyhow as Cloudflare won’t proxy other services. If the proxied services are regular websites then you shouldn’t have issues.