Azaleas & potting mix Q

Hi guys
I'm just interested in getting a better understanding about these shrubs as bonsai with different potting mixes. I've got one small plant in a bonsai grit mix (pumice, moler, cat litter I think or something similar gained from a local bonsai club) and 2 others in ericaceous soil fattening up. Firstly am I right in presuming the one in the grit mix is unlikely to be able to be overwatered unless soaked endlessly? It's getting a daily watering currently in the warm (UK) weather? And secondly how much damage is likely in using hard water fairly regularly. While I collect rain water it doesn't take too long to go through my supply when we have a couple of rain free weeks and I've 6 plants or so that apparently don't like hard water. I'm in a fairly bad hard water zone so am trying to alleviate some of the problem using filtered water but it's somewhat tedious for me as I'm just using my coffee makers Brita filter .

So would help if I knew whether I really need to bother or not. Tks.