"...Any acids like citric acid, yogurt, acidic milk will help keep ph lower...."
Not really. At least if an acid is used in moderation. Citric acid or any other acidic ingredient will not lower the pH of a properly made soap unless you add an excessive amount of acid which will force the soap to decompose and the pH to drop. In that case, the soap isn't soap anymore.
"...You can buy test strips to test the ph of your soap you should be aiming for a ph as close to 7 as possible 8-9 ph is good...."
The pH of properly made soap varies but is often in the range of 9 to 11. If you force soap to have a pH under 9 by adding acids, you don't have soap any more.
Inexpensive test strips used the way most soaper use them are really, really inaccurate. They will tell you the pH is 2 or 3 units lower than the pH really is. So if you get a pH reading of 8 with your test strips, the real pH is closer to 10 or 11.
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edit-- I want to add some actual measured numbers to my comments here to make things more concrete.
Let's say I make a pure sodium laurate soap. (Lauric acid is a major fatty acid in coconut oil). The normal pH of this soap is 10.1. If I add enough acid to lower the pH of this pure soap by 2.6 pH units to a pH of 7.5, the soap will decompose into a mixture of 50% fatty acids and 50% soap. In everyday soap lingo, the soap would have a whopping 50% superfat.
A pure sodium palmitate soap has a natural pH of 10.7. (Palmitic acid is one of the main fatty acids in lard, palm, and the butters.) By the time I drop its pH 1.9 units to a pH of 8.8, the soap will have decomposed into 50% fatty acids and 50% soap.
A pure sodium oleate soap has a normal pH of 11.2. (Oleic acid is the main fatty acid in olive oil.) If acid is added to drop the pH 1.3 units to a pH of 9.9, the soap will decompose into that same 50:50 mix of fatty acids and soap.
Since our soaps are a mixture of these (and other) fatty acids, the natural pH of our soaps will vary within the range of 9.5-11.5, depending on which fatty acids predominate. (I often say 9-11, but that's actually a little lower than what's really the case.)
A high coconut oil soap will have a lower pH due to more lauric acid content.
A high lard or palm or olive oil soap will have a higher pH due to more oleic and stearic acid content.
But no matter what, if you drop the pH of your soap by 1.5 to 2 pH units, you're going to end up with a 50:50 mixture of fatty acids and soap. In other words, you'll have something that really doesn't function as soap. --end edit