THE mods?

In Diplomacy Distant Stars was perhaps the closest thing we had to "en masse". But it has not been kept entirely up to date for Rebellion. There is no matchmaking support for mods, you can play them online over ICO but people who aren't playing the same mod cannot join your game, and there's no easy way to communicate this (save putting the mod name in the game name). However, since no one expects to see mod games randomly on ICO, you pretty much have to arrange to play them ahead of time (a few do have steam groups or other ways of keeping track who is interested).

The Halo mod just released the first playable version, and since it's Halo all of the fanboys are swarming over it (Maybe it does deserve it but it and the Mass Effect mod have been highly ranked on moddb for years, despite them not being playable).

Basically I would just find a mod you like and post to see if anyone is up for a game. This is my favorite list, otherwise just say what you're looking for and I'm sure people will give you recommendations.

En masse? you mean online? I think almost noone plays mods online, as it is very rare to see games of mods, and they remain empty until the host gets bored and leaves.. Mostly I think Star Wars mods are preferred (I only saw these mods more than once online..) but for me none is at an acceptable level, although many like them, maybe you can have players for even a 2v2 at the weekends. Probably what Goa said, you will need to join a Steam group or pre-arrange the game with your friends if they are playing this game, if you want to play mods against humans.

Try Goafan77's mod, the Enhanced 4x https://forums.sinsofasolarempire.com/414639

I couldn't really find a mod that is really playable and enjoyable, but this is the closest to it, and the Best mod I think. It enhances the 6 factions in a great way and changes game mechanics slightly, in an enjoyable way.

There is the Maelstrom Mod for all versions of Sins of a Solar Empire too, I liked it, but for me still lacks many things I can't really form into words, but the best mod featuring new factions in it, but it seems somewhat finished, not like reusing almost everything from the original races and such, although the Replicators are somewhat beta versions, and another release is due soon with improvements I think:

https://forums.sinsofasolarempire.com/385873

It is not hard to find other mods, like the Star Wars ones, or the Sins Plus mod that adds new planet types and planet bonuses, just a bit of proper Google use and voilá. It is really up to you, what kind of changes you like etc.

Sins of a Galactic Empire has a decent MP following; we usually play pick up games (one person mass-messages the Steam group to see if others are willing). Over the past number of weeks we've played at least a dozen games.

What exactly do the Mass Effect/Star Wars/Halo mods do? Can't really find good descriptions. Thanks for all the info, will check out links!

Add Mass Effect/Star Wars/Halo playable factions. Some of them might be "total conversions", in which case the original factions are taken out, and likely change a lot of other things as well, but the main thing is the new playable factions.

just be aware partial and total conversion mods sometimes take away good features of this game like shield mitigation and such, making it weird.. and many feel like incompleted as so many things are re-used from original factions and even the ability button is the same, but sure every mod worths a try

I highlight which mods are which in this thread https://forums.sinsofasolarempire.com/423212.

You essentially have 3 types of mods.

Total Conversion - Aims to replace all assets from the original game with new ones to fit their universe. Sometimes a long road to fully replace all vanilla assets.

Expansion - Aims to add additional playable content to the original game. Could be new races/factions, but also could be extensive gameplay enhancements like E4x.

Graphics/Effects - Aims to improve the overall look and feel of the game. This type is typically included in TC or expansion mods. 

Thanks again for being so helpful. Do the visual only mods(enhanced planet textures etc) still allow the normal multiplay since all it is affecting is my clients visuals? 

In theory yes. Any mod with a "Checksum" of zero can be used in multiplayer. I always use a music replacement mod is MP for example that just loads a different set of sound files for example. However, for both textures and sounds, they have to be set up in such a way to "Replace" what is already in the game. If I tried to add a new sound effect for example I couldn't use that in multiplayer, even though it doesn't really affect the gameplay balance at all.

I have only seen maybe 1 planet texture pack ever released, most planet mods add new planets, and thus cannot be used in multiplayer.

Bailknights Enhancement mod is giving me a checksum, but as far as I can tell it replaces everything. Any ideas?

No it doesn't, at least not the Rebellion one I saw. It changes the ships to tell them to add new particles (Rebellion added new particles, so to get it to work with Bailknights the guy who updated it had to change every ship to use the old ones). You can try deleting the "GameInfo" and "Mesh" folders of any mod to get rid of the checksum, but they may not work as you think (when I tried it with Bailknights, I only noticed the explosions showed up).

Easiest way is probably to run a comparison program between bailknights mod and the normal files, then take all of bailknights stuff and manually transfer it to a place where it will not affect multiplayer