Which Mike, and what does their manual and specs suggest? It probably doesn't matter because virtually all currently available non-brass Mikes are likely to operate well on 18" curves...by design. In the same way a Trix Big Boy can operate on typical EU 18" radius curves, a N. American 2-8-2 should work very nicely on 18" radii.
I assume you are thoroughly familiar with laying trackwork, so you understand that to keep things reliable and fun, you'll want to take some extra pains to ensure nice and proper joints, and that your tracks will lie on a solid platform. If your joints allow changes in grade from piece-to-piece, especially along a curve, you'll have more derailments. Maybe not tons more, but enough to impact your enjoyment and confidence. So, if you do this thoughtfully, as you seem to already be doing, I don't think a currently available Mikado is going to give you many problems.