I tend to use topknots from extra CSM heads for my flamers. When it's painted up well, it looks great. Remember that flame is hottest at the center, so you actually want to start with white or yellow, and highlight out to red (which feels wierd just because you're getting darker as you go out, unlike what you do with most highlighting) but it's more realistic.
I'd try to keep your bases fairly uniform across the army. If you want to do a lava/fire-themed base, go for it, but I'd repeat it across the army so you've got a consistent theme and appearance.