Ever since we escaped from “me.com” one of my clients has been repeatedly advised by Apple support that multiple accounts on an iDevice will not work. I point of fact, multiple accounts on an iDevice work fine – assuming the user does not try to “optimize” according to Apple Advice. And, multiple accounts are not limited to just iCloud accounts. I have decades of experience with other email provider accounts co-existing with iCloud accounts.
The first rule Shamino provided is critical. Each primary user of an iPhone or iPad (or macOS) should have a personal iCloud account associated with that device. In the case of an iPhone that includes the appropriate phone number. This all provides correct routing for calls and texts, as well as proper “Find My” device location.
For sharing a common Contacts list, as may be done by a couple, one user must “own” the primary Contacts list. The other user must use a secondary iCloud login with “Contacts” checked and preferably with the primary iCloud “Contacts” unchecked to avoid inadvertent entries in the unused Contact list.
Apple Notes sharing requires all desired sharing users to have their own iCloud account. Any user may create a Note and share it with any other iCloud user.
Mail accounts should be primarily used from the “owned” device. There is no defined limit on the number of additional internet accounts that may be added to each device. But, unless you want to confuse users, for the most part only email should be enabled for the additional accounts.
For some items like Contacts and Calendars, one could choose Exchange or gmail. In most cases it reduces confusion and probability of “lost” data if only one vendor is chosen for these functions. For mostly Apple-centric usage, iCloud should be the choice.