I want to pick objects of variable height so the actuators need to be on the side. There 5-6 standard objects that I want to grab at the moment, very light 150grams maximum, ranging from 8-13cm length. What kind of designs? I have looked at various grippers but they were either too expensive or didn't exactly fit my requirements
Things to pick are very light, under 150grams, and the gripper walls are flexible so I can simply squeeze 2mm tighter than product dimensions. I've tried it didn't have an issue with this.
The main benefit is, it does the job, it's cheap enough for a prototype and it's simple enough for me to make. 
I am a software engineer, I don't have much experience with hardware apart from hobbyist arduino stuff.
I have gotten a quote for this from FUYU
It fits the requirements but it's not the lightest of components at 1kg. Still with two of them well below the 3kg that the robot arm is able to handle comfortably. Would be nice to find something around 750grams but I think this will have to do for a prototype. Costs about 50, the actuator itself, around 300 if I purchase driver, controller, power supply etc
So it's a gripper. Parallelity of the 2 moving plates has to be ensured or is just an assumption I take from the drawing?
Anyway, easiest is a RC servo with some spring mechanics for load adjustment. You find mechanical solutions for this in the reprints of steamage mechanical books - don't need linear rail for this 
Note that you should do the maths on the necessary holding force first. these linear rails with threads deliver magnitudes more force than you are looking for.
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They have to be parallel but not strictly some slack of 2-3 degrees is acceptable, I don't mind if the items are not perfectly alligned with the linear rails.
I thought servos are not recommended for full 360 revolutions? ok thanks for the tip I will look into mechanical steamage books, although I can't see how it can be done without a linear guide of some sort.
You don't use them to drive threads. You use ~ 120° rotating angle to do the full gripping action.
Yes, but get one with a decent mechanic and at least with a 30kg servo. And an adjustable spring - or better with some kind of force sensor. e.g. HiTec has some RC Servos with adjustable maximum force at ~ 120€
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