I can only comment upon size comparability, not whether a particular range has what you desire. Magister Militum, Peter Pig and Steel Fist marry up with Epic. Some Freikorps will, but size varies wildly within ranges – foot are more consistent and should be okay, mounted figures are very hit and miss. Naismith were an almost perfect match, but they are out of production and what sources were available have practically dried up. (Of course their horses were a bit odd style wise, working and looking so much better on Peter Pig horses)
Essex are a head taller and look very incongruous next to Epic style wise. Minifigs are generally a head taller than Epic, but are better stylistic match for Minifigs can be a bit hit and miss size wise, some are very much 'true 15mm' many are not.
Epic are very much in the bracket of 'true 15mm', so those ranges that are 'true 15' are your best starting points. I am very familiar with ECW ranges, just not with what you are asking for. Just because a figure range is 'old' doesn't automatically mean that they are 'true 15mm'
Of course if you want to see side by side comparisons, so you can make your own mind up, have a look at my Epic posts over on KeepYourPowderDry
For lancers, would Peter Pig ECW Scots lancers work with headswaps for morions? They'd be a very good match size wise with Epic cavalry
