ph fluctuations would occur if nitrification happens, but you will not allow this to happen as you will do regular water changes. there are many ways you can use peat to alter the hardness and ph
1. placing a bag of peat in a cannister filter and allowing the ph and hardness to drop as water constantly passes through the peat. every month or so you will have to replace the peat, it all depends on the quantity and quality of peat. if you do small water changes say 10% regularly, you will not have to treat the water with peat as the volume of water replaced is not significant enough to cause an adverse effect on the ph or hardness, thus eliminating fluctuations. experinment and see what ph and hardness is achieved with different quantities of peat
2. not placing peat in filtration but treating the water with peat before placing it in the tank. you could do a weekly 20-30% water change with water that has been treated with peat, this would be done by leaving a bag of peat in a seperate container with over week. ofcourse water could be prepared faster through the use of a filter with peat
personally i would go with no. 1
ofcourse ro would be the best investment, personally i would not invest in one