Welcome to Librarium Online!
I am planning to run two squads of six in my army but I find that their numbers leave something to be desired so I was wondering what you would replace one or both squads with.
Warbikes combine speed, decent shooting at close range, and their PK nob. If you want more shooting roles filled out, a unit of 20 shootas with 2 big shootas in a shoota-equiped battlewagon can provide a ton of fire and a strong troop choice that still boasts a nob's powerklaw. Stormboyz are real fast and can pack a big punch. Sluggas in a trukk can deliver a PK Nob plus a bunch of boyz' attacks. If you just want speedy shooting, buggies equiped with rokkits or big shootas, or deffkoptas, can do that.
I personally like warbikers as they are very versatile and fairly survivable. The key is using them in situations where they can survive. Spending a turn or two shooting with them before charging in to help with another combat, attacking vulnerable tanks or shooting squads, or suiciding and holding up an important enemy squad are all some uses. You need to be careful with them, but they are extremely varied in their uses if you're clever.
I agree with Seanchadith's suggestions.
Since the main problem you have is numbers, the best option is either shoota boyz in a battlewagon, two units of trukkboyz, or a single large unit of stormboyz.
"Any job worth doing, is worth doing with a powerklaw."