Just note that not all costume parts are available in all the platforms.
Specifically, Icon can use parts not available elsewhere.
Levels of Validation(Might be incorrect, subject to Internet correction.)No Validation - Icon NPC Mode - Virtually anything goes parts-wise, even if it's 'breaks' the display (like Hamidon in Atlas Park.)
Some - Paragon Chat (DB Edit allows SOME parts from NPC Mode, but trial and error)
Some - Icon Normal Mode - Limited to Costume Editor/Icon Parts, but the list is expanded from live
Full Validation - Live Game (City of Heroes Issue 23, 24, 25.)
-----
Validation is the game keeping NPC parts unique to enemy groups, archvillains, and "IP Characters" such as Trainers and Contacts out of the pool when editing a player costume. Validation also keeps problem costume parts out of the hands of players, or fixes issues with textures (like that one time where miniskirts were rotated 90 degrees so 'slits' were on wrong.)
If your costume doesn't validate in Paragon Chat or Live, it's the same behavior in the Costume Editor: you'll see an error message when you load the saved costume that some parts failed to validate. Click through, and you'll be given an option to "Attempt Costume Fix" for the Costume Editor to pick alternate parts before allowing you to proceed with the costume selection. You can change the parts, of course, but the non-validated parts will not be available to you.
In Paragon Chat only, after DBEditing a costume, a lock icon will appear on the affected costume slot if the costume doesn't validate. That means visiting a Costume Editor and changing the costume is a one-way transaction. Once opened in the Costume Editor, you can't put the costume back the way it was. You'll get a warning before this happens. (This applies only to Paragon Chat because DB Editing the Live Games as a player is impossible.) The lock icon will still appear on Live Costumes in case of Costume Part mistakes, or when a part gets deleted.