It is ok to mix the tap with RO.
Where I live my municipal water can come from wells or a river. It is very good when coming from the river <100 ppm and only chlorine no chloramines. When coming from the wells it is well over 250 ppm at times still no chloramines, but the alkaline is hard to deal with. I have to check each time I fill my top off tank to determine what water I am getting.
Adding a small amount of Ca & Mg to RO water can bind a bit to the silica but it is such a small amount it does not cause a loss of the silica and if Ca &Mg is full bound to the RO even less can bind to the silica.
RO water costs in filter time and waste water, if your tap water is probably ok to use at 155 PPM, I would use that. Check your water reports for chloramines.