To be extremely explicit - you do NOT have to place your infusion reagents around the altar as depicted in the thaumanomicon. The thaumanomicon depicts the reagents arranged radially around the altar (with a clear north) but this depiction has no actual bearing on the infusion.
It is quite valid to simply place a set of altars to the left and right of your infusion altar (without regard to absolute north) and mix and match the reagents on either side.
Also, once an infusion has started, you can place extra items on pedestals and they will not harm the infusion. This is especially useful as you can balance out infusions with odd numbers of reagents, by placing a random reagent on the unbalanced empty pedestal after starting the infusion.