I have just taken the plunge and purchased a couple of very nice Egares Helldorado boxes, great figures.
Is there any problem with using "normal" round bases rather than the "display" type bases, I guess this only comes into question if I should ever decide to play in a tournament. The rules make a great deal about being sensible and just slapping each other in the face if players disagree on rules, so something a minor a one round base verus another would seem a small point.
The main reason is that these models will fit in very nicely with the rest of my generic demonic army and I would rather have them all on the same style of bases.
The size is probably important, although to be honets, a couple of lmillimetres heare and there isn't going to break anything.
However, Em-4 Miniatures make 30 and 40 mm flat round bases (same style as the GW ones):
http://www.em4miniatures.com/acatalog/MINIAATURES_BASES.html
If you find anyone who makes 50mm ones, give me a shout. :)
I`d hazard a guess that the same size would be fine, but a different shape would be a no-no.