No probs. It suggests basing Open Order (OO) mortals on round bases and Close Order (CO) on square, but the reality is that the close order simply have to rank up and the open order do not so base shape isn't really an issue.
It's also why I like the idea of 15mm or smaller on similar sized bases as you can probably depict CO and OO much better in this way.
That said, I am going the 28mm route because I have some Romano Brits and some Vikings already to hand to do battle! Just need to find some Gods!