First, I know it's my game and I'm the GM and I can rule how I want. :-)
With that, help me work through this. Two separate issues.
First, Horde/Squad attacks in general seem to need Spread to represent the number of attacks being made. So that's an easy house rule. If a Horde/Squad has 3-4 members, all its attacks gain Spread +1. If a Horde/Squad has 5-6 members, all its attacks gain Spread +2. (A Horde/Squad with just 1-2 members gains no Spread...too bad, so sad.) Done.
Second, what if the Squad Leader has a different gun than the rest of his squad? FOr example, Centurions often lead Necromutant squads and Necromutants often lead Undead Legionnaire squads. Yet they use different weapons. My solution is to have the Leader attack separately as an Elite with no real bonus. The benefit of the Squad is that the leader then may direct his Squad to attack either the same or another target as a Free action. The rest of the Squad then attacks normally, rolling 1d20 per member and benefiting normally from Let Rip or DSP spends as normal. As above, this attack automatically gains Spread if the squad has enough members.