Unfortunately, only those who also have purchased DLC will see your custom outfits. I only get to see the default A costume on people with custom outfits - though I can see they are using one because it will display, "Costume D" or "Costume C", etc.
Which really sucks - almost like Sega is punishing those who have not bought the costumes, while inadvertantly devalueing their own DLC to those who have.
I'd have loved to have at least been able to see people's outfits - it may have enticed and amused me all at once; but 15 bucks is a lot to spend to play dressup on half the roster - let alone the whole crew. Especially when only others who've also made the choice to buy can see your custom dude(ette).