I'm just finding out all about ph myself -
What I've found out so far is that yes - fish can adjust to the ph of your tank (depending on the fish and what they are used to) and it's more important to keep the ph steady, than to adjust it to a particular number.
If you do need to adjust the ph, it's better to do that through adding something to your tank like peat or driftwood, rather than trying store bought treatments like ph down. That way you are more likely to succeed in keeping a steady ph. I would guess though, that you are going to struggle to lower that ph, even with the methods I mentioned. It may not be worth it, rather research for fish that do better in harder water.
You don't need to add salt. People used to believe it's necessary (and some people on this forum will contradict me!
) but apparently they've tested this and found that there is no benefit to adding salt. Only use it for treating specific problems, e.g. as a medication, or if you want to keep brackish fish.
This is a great guide to how to cycle your tank, in case you've not found it already.
The (almost) Complete Guide and FAQ to Fishless Cycling - Aquarium Advice
I don't know about your filter question - hope somebody else on here will answer that one.