Outdoor tap from cold water pipe to boiler ?

Yes should be ok but better direct from mains supply

Yes that's fine, the cold supply to the boiler is from the water mains

As long as it’s on a cold water main you can connect.

You're combi boiler is fed off the mains supply just like all taps/toilets in your property. The rising water mains comes in to your stop tap, is generally drawn off in 15mm pipework. It makes little difference where you tee off for an outside tap. With normal, or average water pressure it makes no difference wether a tap is opened, a toilet flushed or the outside tap being opened. All of these will affect the flow from any hot water being drawn.