This answer mentions that the ideal battery percentage is around
70% - does this mean I should keep my laptop always plugged in with
the 60% cap?
It's a good answer that tells you most of what you need to know, but I will add a few comments:-
There are two main 'wear out' mechanisms in a Li-ion battery - damage caused by the stress of charging and discharging, and damage due to oxidation at high voltage and/or temperature. So for longest life you should maintain a low SOC when the laptop is on mains power, and also try to keep discharges shallow when operating on battery power when practicable. If you need to run on battery power for an extended time, charge it up just before you need to use it, then recharge to your desired normal SOC afterwards.
If you normally run the laptop on the mains then avoid doing it with a fully charged battery for long periods, as both the voltage and temperature will degrade it faster. But what if you often need to operate on battery power, and need the full capacity to get the required run time? In that case - just do it. The battery will eventually wear out and need to be replaced, but by that time you might want a new laptop anyway!
Manufacturers of devices that need a long battery life (eg. laptops, cell phones, electric cars) often adjust the 'full' and 'empty' points to get more cycles and longer shelf life. So it may be that when your device says 100% SOC it's actually only 80%, and when it says 0% there is actually 20% remaining. In that case it could still pay to keep the average charge at ~50% with shallow charges and discharges, but the chances of premature battery degradation are reduced even if you don't.
Battery technology is constantly improving, so a modern chemistry may handle higher SOC better. Therefore if the manufacturer of a particular device recommends a certain charging strategy for best life then you should follow it. But that strategy may not apply to other devices, whether they use Li-ion or other battery chemistries.
I've also heard that a battery should never be plugged in and that it
should always have charge and discharge cycles for optimum battery
That sounds like advice for Nicad batteries, which suffer 'memory affect' if continuously 'trickle' charged. Fully cycling a Nicad battery minimizes memory effect and can even restore the capacity of batteries that suffer from it. That doesn't apply so much to NiMH batteries, which are often continuously charged in cordless phones and other devices that spend most of their time on 'standby'. Lead acid batteries should be 'floated' at close to full charge to avoid sulfation.
So advice for a different battery type could be exactly the opposite of what you should do. Your G15 laptop has an option to cap the battery charging at 60% for a good reason, so you should use this feature if it is convenient.