You can do the first one by reducing all psionic attributes on your units and the aliens alike. The attack strength decreases by distance in a linear fashion, meaning if your psi attack is cut in half and your target's psi defense is cut in half, your psionics are just as effective when you're next to the target, but any further distance has twice the reductive effect.
Also, reducing psi attributes across the board will make psionic success more random. There is a specific range on the random number roll. The lower your psi attack rating, the more the random number matters. This means your psi weaklings might be successful if they try enough times, while it also means you might never be completely safe from enemy psionics.
Inversely, if you increase psi attributes across the board, success is much more deterministic while range becomes less of a factor.
If you decide to change psi attributes, I recommend you change only psi strength values, and change them all by the same percentage. If you cut every unit's psi strength exactly in half and leave psi skill the same, the net psi effectiveness across the game will be essentially the same as before, albeit with the changes discussed above.
