Here is my opinion. Firstly, I've never used Pro-Mix Organic, but I love me the Pro-Mix potting soil.

Don't add anything. THERE! I said it! Seriously, I like it just like it is. I may add 15% worm casting or bat guano if I have some laying around just to give the seedlings a little taste, but it really isn't needed, you just get on the nutes a few days earlier.

Lime is more properly called a PH stabilizer rather than a PH raiser because no matter the PH of the water you add, it tends to keep the soil at 7.0. Lime is basically calcium carbonate which reacts with acids to neutralize the solution. If you keep pouring acidic water into the soil, eventually the lime will run out and then you got trouble. Much mo' bettah to PH your water first and not rely on your lime to fix your PH which could run out whenevah and you'll only find out when your plant starts dying.

But here is the deal. Lime is already in there! They know what they are doing! You can add more perlite, but in my opinion the soil is hard to water properly then. It gets drainy and you almost have to water every day.

Hope that helps.