With regard to Forgeworlds models, a lot of them get rules released on the Forgeworld site as experimental rules. From there, they will get included in different books .. Imperial Armour, the Horus Heresy books and so on. Some of them will get rules updates saying 'This unit can be taken in codex X as a unit type Y'.
There's also the ability to proxy or 'counts as' a unit. For example, I have one of these
. It's not in my Chaos codex, so I can't use it. I can say that it 'counts as' a Vindicator, so I can still use the model.
Or I can use the Space Marine codex instead of my Chaos codex and use it there. My Marines are newly turned to Chaos, so while they look different to normal marines, they still have the same rules and equipment.
40k gamers, in my experience, are quite happy to bend the rules in order to justify fielding the things they want to. When that's done purely to powergame, it's not so nice, but if someone wants to do it for fluff reasons, I'm unlikely to have any objections.
Almost all the models that have been out previously are still viable. A few special characters have disappeared, but if you still have the model, you just field it as a standard commander or what have you.
I don't play Warhammer Fantasy (WFB) much, but I understand it's been hit a lot harder from time to time when army book revisions come out, and entire regiments of models are now no longer legal.
There are a few other games along the lines of 40k.
Infinity is one that plays at a somewhat smaller size, from what I understand it, with a smaller model count on each side. Some very nice models too.
Warzone has just been redone and rereleased with a new rules set and new models. It'd be similar to 40k in terms of model count, and has some models that could swap over quite nicely with 40k lines.
There used to be a Starship Troopers game, but it's out of production I believe.
That's all I can think off, off the top of my head.