Are custom skins/models visible to other players?

No, they are not. However, different models may cause mismatches because the game calculates real distances from spawn points, and different models may have different spawn points/in different positions attached to them.

Pure texture changes are always safe, however.

Editing voice files AND editing "Voice.ini" won't cause mismatch as well.

What exactly is a mismatch?

Mismatch is the state of the game when synchronisation between the players is lost, often due to some error in the code or the connection. This sadly ends the game, along with a big, black box with small white, text that says as much, and not what exactly went wrong.

Pretty much to reduce miss matches, play with less ai although mizos should be working far better now play on official maps, and play with the send delay button on.
in terms of addons, as long as you both have matching files (so all your addons are the same) you should be fine for any sort of multiplayer additions

Those aren't really allowed are they?

Send delay button? What's that?

The send delay button is in the options screen near the bottom.

As for mods being allowed, if its a small balance patch or its adding a new unit here or there, sure its all allowed, but if its a whole new mod (like you play contra or vanilla ZH on swr net) thats not allowed.
The testing team does their testing with changed ini's on swr net, and ive played a couple games of the mirage tank addon on there too, so i dont see why it wouldnt be allowed.

Only if the other human player have the skin/model.
If you change the properties of any unit, will cause mismatch and/or ban.

We already outlined above what is possible and what isn't.

This is confusing me.

Here's what i can get. Correct me if i'm wrong.

Multiplayer addons are possible as long as all players use the same INIs, but only testers are given access to addons on swr.net and non testers can play only vanilla swr mods or risk being suspended/banned.

Is this right?

Modified INIs are forbidden no matter what or who, they can get you banned. Additionnal INIs, like Modified UI or WidescreenUI are tolerated last I remember. The goal is that you shall not use different stats. Graphics is OK
You can use modified art assets, since they are not INIs, but tga or mp3 files in your Art folders.
If you do use such add-on, DO NOT report ANY bug, since they are not officially supported by the team. No Exception

Not quite mael, things like Zekes Mirage tank mod and small adjustment mods are allowed so long as it is still rise of the reds, however you are at risk of getting picked up by the auto-cheat.

You are 100% right (and i feel silly for not mentioning it earlier) that id you use any modified anything (including textures and widescreen adjustments) do not report bugs under any circumstance. Only report bugs used in the vanilla game.

How do you replace game music with mp3 then that would be really nice.