Four Marksmans is a good core group. I'd add a single bodyguard, preferably something with decent-sized LB-X cannon. The artillery can make short work of a lot of attackers, but a versatile bodyguard means more situations where he can take care of things by himself without interrupting the Marksmans' fire missions. Besides, he'll have backup in the form of the SPGs' turreted firepower, and four Marksmans is a fair chunk of gun.
For the sixth unit, I'd go with some kind of scout spotter. I'm not talking about someone to spot for regular fire missions, I'm talking about directing fire against inbound attackers. If you have Copperhead rounds, anything with a TAG will do. If you're playing Alpha Strike, advanced sensors help spotters, and spotters can help the first shot and onwards, unlike TW/TO rules where they only help agree seeing where the first shot lands. Given your likely faction, the Beagle Hovertank may work very well here.