Play with the published rules on the GW website and you can't go wrong.
Alternatively if you used the Epic UK rules no one would be concerned.
http://epic-uk.co.uk/index.shtmlAs to the rest its optional. I normally played with the online changes in the rules forum (its an A4 sheet covering all the published Epic armies). Certainly most of the players in London do if they aren't practising for a tourney.
Online changes
http://www.tacticalwargames.net/forums/ ... 69;t=13201Soon I think there will be a few tweaks to the core rules as well.
But certainly as long as everything comes from the same source - the published stuff ont he GW site, the Epic UK site or the rules forum here its fine. Its all balanced, just in different ways and there is nothing stopping different armies from each playing each other! (Edit, in some cases we are talking making a difference of +/-50 points to the average army.) Just know what you have, where its from and what its stats are (and tell the opponent before hand!)
The playtest lists here are just that - playtests. This is actually the playtesting space marine section of the site, though it tend to get used for other marine related stuff as well.
So for instance there is a list at the top of the sud forum detailing all the chapter lists people have thought up. If you want to play one, go ahead, nowt will happen to you. Note how the battle goes and anything which is too good/bad and your thoughts on whether or not it delivers and report back.
I suggest you start with Salamanders

, though not against Rug or Ginger as they I suspect must be sick of them by now

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