Based on signals generated by SPX I want to simultaneously open positions in each of several other symbols. When I open such a group of positions I want to allocate a pre-determined percentage of available cash to each position (e.i. 40% to ABC, 15% to XYZ, & 45% to DEF). By reviewing some of the symbol-rotation strategies I think I've got a handle on most of this, except for the position sizing. Can I code this position sizing into the strategy code or will I have to compile a new PosSizer?

You hit the nail on the head - a new PosSizer is the way to go.