[pothole]

I'm not sure I'd be too worried. It's certainly OK to be driven to a garage to have the coolant system flushed. The only possible issue is if whatever you put in makes the coolant more conductive in the electrical sense which over long periods of time can cause problem involving electrolysis and the alloys in your engine. But that takes ages. A pint's not enough to make any different to the cooling efficacy.

I'd probably have the coolant flushed. Not even sure it's worth doing that unless you do it yourself - I doubt if you took it there yourself they'd flush it properly, they'd probably just pull the engine plug and then refill.