Shivans are extremely poorly suited to ground combat by the very short reach of their weapons.
Two ideas on shivan ground combat suitability:
1:
I'm sure that ANY species that has an integrated, shoulder-mounted beam cannon has the technology to change the focus of the laser with a simple thought. Think of it like your energy management system. you've got 3 attributes, all of which require that the other be turned down in order to increase that attribute. The three attributes of the Shivan shoulder-laser would be:
Width: The field of effect of the weapon.
Range/Power: The distance at which the laser is still instant death to a human.
# Of Shots: How many shots the laser can do if the energy source is not refilled in any way. (in reality, the laser power source is probably recharged just like your weapon energy, but for the purposes of this definition, we'll ignore the recharge factor.)
I'm sure that, for the hall fight, the shoulder lasers were set to maximize Width and # of Shots, which would be standard operating procedure while in a firefight inside a shivan ship. The lasers would easily wipe out multiple hostiles at close range, while the increased # of shots allows the shivans to continue fighting for a longer period of time. Range/Power would not be nessicary at the close ranges inside a ship, and so the lasers wouldn't burn through the hull or damage critical parts.
For ground fighting, I'm sure the shivans would minimize Width and increase Range/Power, to have a more effective sniper-like weapon, then once they closed with the enemy, they'd revert to the above configuration. This allows the Shivan laser to be more powerful, more useful, and more true to the shivan mindset. Do you think the shivans will settle for a SMALL, NOT-VERY-POWERFUL shoulder cannon? We've already seen that they LOVE giant beams, and will always try to be as deadly as possible.
2:
I've always pictured shivan ground combat much like the movement of a Starcraft Zergling, except exagerated. They would move across the ground in huge, flying leaps, beaming as they go. Each Shivan would launch themselves at a group of Terrans, land either near them or in the midst of them; then let loose with beams until all nearby Terrans are gone; then launch themselves into another group of Terrans.
Well Carl, am I at all close to correct?
Oh ****. I've talked myself into making a cutscene that features Shivan ground combat.

I've got a place for it in my mod, but I'll wait until I actually finish my mod before I make the movie and release it as an add-on.
Speaking of my mod, I've already got all the details of the ground combat methods of my new species all worked out. I think the cutscene will feature them as well.
Anyone got a high-poly Shivan, Terran, or Vasudan model? I'd love to use it in the cutscene. I'll give you credit. Just pm me if you're interested.
EDIT:
Ah human nature... you'd think with an upbringing like that the human race would be better fighters in the FS universe
you have to remember, in FreeSpace 1, we had just gotten into subspace travel, and we didn't have much ability to test space weaponry before that. Frankly, we did pretty well, considering we had little in the way of experience.