Really Abetillo has covered the main points, but for Epic I always use the same advice I read an awful lot where historicals gaming is concerned - it's a really big issue there because for some periods of history (ww2 or Roman era for example) you have so many different companies making miniatures scale does become an issue. Perhaps the same thing has happened now with Epic; you have original GW, AT, Vanguard/Onslaught miniatures and even now 3D printing.
The advice for Epic is the same - generally changes of scale on the tabletop can be acceptable but only if you do not have mixed scales within a unit or next to each other. So if you want to use Warlords, use either original GW or AT. Don't mix them up, because the scale change is so egregious that they will look completely out of place next to each other. For that reason I wouldn't use AT Knights and original Epic titans either, because a new Knight can be bigger than old Warhounds!
With infantry and tanks its a bit less clear cut. I do use some old GW stuff alongside newer Vanguard miniatures, because the difference between 6mm and 7.5mm is not really noticeable from the tabletop. My only rule for that is I don't mix infantry type for Marines (as the difference between a GW and Vanguard marine is noticeable on the same stand) - but again its not a hard rule, for Orks I mix in a lot more stuff, as Orks themselves are much less variable in size.
Finally - I have found this kind of thing is really subjective! Some people cannot stand even 1mm difference between miniatures and find it upsets the look of an army. Other people quite happily mix scales, and sometimes you have to just use what you have! So YMMV
