Man, having that many more units makes for a much longer game. I can't imagine how long this would take with the regular rules. Too long, I think, even though I'd prefer the ability to remove cohesion hits and rally routed units (you can't do either in Simple GBoH).
I decided to start with the Roman right flank cavalry in it's historical position on the far side of the river from the rest of the Romans, and started the game with their surprise attack on the Tarentine left. I chose to have two of the units go after the two archer skirmishers within range, and the remaining five disrupt Pyrrhus' heavy cav. They managed to flank two of the units, which both flubbed their reaction facing rolls (and with TQs of 7!), and both took heavy damage, although not enough to rout them. Pyrrhus responded by encircling them, and I suppose the Romans should have protected their cavalry's flanks instead of going after the two skirmisher units, which was mostly a bust anyway. The result was a complete rout of the Roman cavalry on that flank, but I did manage to get Pyrrhus and three HC units to pursue off of the board, which could be very useful as he's the OC for the Tarentines.
Next, I brought up the Velites to throw javelins at the Tarentine skirmisher line, which didn't go as well as I'd have liked. While they did some damage, they also took some damage, although I screwed up the Hit And Run Tactics rule a bit, which assumes that they run forward one hex (which was into the river, which affected their throws), and then back. In the end, they lost a couple of units with only one Tarentine loss, although their skirmisher line is largely useless as a fighting force at this point.
The Tarentines used a few activations to get their phalanxes, medium infantry, and heavy infantry into a longer line that matched the Roman line, and formed up poised to charge once the Romans crossed the river. The Romans responded by bringing up the Hastati and Principes behind the Velites, with the idea that they'd get across the river, then take the Tarentine charge by shifting their stacked legions into the empty spaces. Unfortunately, Pyrrhus had by now managed to bring his HC units back onto the board and they have formed up to challenge the flank. The force is down to five units, as two were too badly damaged to risk, but it will still take a few Roman units to deny that flank, and I have no idea how they'll prevent them from just rolling up the line once everyone is stuck in.
At the end of my play session, the Velites had forced the river with little success, and have begun to pull back. Unfortunately, most of the units were out of command and couldn't retreat easily, so I expect to lose a few more. At this point, I've lost seven Roman Cavalry and three Velites for a total of 50 rout points to the Tarentines losing a whopping 2 for their lone archer. This isn't as bad as it might seem, as the Romans can take 185 rout points to the Tarentines taking 120, but the margin of error is much tighter than I'd have liked for it to be.
On the bright side, the Roman Hastati and Principe lines are both exactly where they should be, and the Velites created just enough confusion with the skirmisher line that the Hastati can get across and prepare for the Tarentine charge. The scary part is that the flank with the Tarentine Phalanx units is the same side as Pyrrhus' heavy cav, so I may keep that side of the army on it's side of the river and engage the less dense units first. The risk is that the elephants are behind that flank and there are special "Scary Elephants!" rules for this scenario (it's where the Romans first saw the beasts in combat, much less at any time).
I should have the second part of this report up and the scenario finished in a few days.