Charging a USB type-c equipped MacBook Pro with a non-Mac charger - would it harm the battery? I've realised that I'm able to charge my new USB type-c ported MacBook Pro using an iPad charger (with a 3rd party cable) and also I can use my old Google Nexus 5X fast charger which has USB type-c port too.

Using these charges keep my laptops battery on level - no charge, no discharge.

My question is, if I keep using them, would they harm the battery?

Update:
+ 1 question: What's the case when my power supply is just below the amount required by the system? I mean, the slowly gets depleted. Could this extend battery life by reducing cycle count?
There are two questions here:

Will using a non Apple USB-C charger harm the computer/battery?

No, as long as the charger is from a reputable supplier there is no reason to expect any physical harm from using other brands of charger, power is power, and the MacBook will only accept the amount of power it needs to fulfil it's need.

Will keeping my battery topped up at all times, no charing/discharging harm the computer/battery?

Yes and no, this is a particularly complex subject, but sufficed to say your battery like a person needs to remain active to be healthy, but equally overdoing it is going to wear it out prematurely.

So your device has a rated number of battery cycles, typically about 1,000 on modern devices. A cycle is one full discharge and recharge in however many little bits that takes, so limiting how much you discharge and recharge the device can prolong the life.

However the battery needs to remain active, and keeping it fully charged all the time can do harm as it's constantly keeping the battery topped up...

There is a really good article from Apple which I don't want to reproduce here that you should read:

https://www.apple.com/batteries/