It doesn't matter if you use a Label or RichTextLabel as a parent node. They should act the same in this case. If the counting doesn't start, did you connect the signal properly? And did you check "autostart" for the timer as well?
For the timer to stop when the player collects ten coins, you should have the node that manages the player's inventory send out a signal to the timer that stops it. I'm not sure what you mean by "display it", though. If you mean display the timer, as long as you don't hide the text node it will be displayed. If you mean the number of coins, add another text node and update the number in it similarly to how the timer's text is updated, only except when a coin is collected and not every millisecond.