The hydro nutrients might work. It's just that you're bypassing the soil to a large extent. You're missing an oportunity to feed the soil microbes (the stuff hydro growers don't feed). I don't think it would "damage" the plants. And, certainly not 1ml. :)
I think your problem is probably
Happy Frog soil being dense/heavy, slow drying. I've read about others mixing it with coco to improve that condition. If I used it, I'd mix 50% perlite into it.
So, I wouldn't flush. That would be excessive for that small plant. Especially if you've been treating the soil like a houseplant (adding water occasionally instead of letting it dry). If this is your first grow, you really will be amazed how dry the soil should be before watering again. Get in the habit of lifting the container, and judging the weight. When it's ready to water again, it should feel alarmingly light. It's really hard for a new grower to appreciate how light that is.