Hey Checkers...
I have used this source of Lacto to inoculate my kettle sours with good success. While by no means am I a pro in Kettle Souring, with three batches under my belt. All three have come out really good with no detectable faults or off flavours. So take my steps as some info, and not gospel! My technique was pulled together from various online articles, podcasts, Milk the Funk wiki and discussions with Pro Brewers who kettle sour.
I recently posted this on another thread regarding Lacto...
http://aussiehomebrewer.com/topic/89263-lactobacillus/?p=1407032
In regards to what you have done...I think that 23c is too cool for kicking this Lacto off. From what I have read Lacto likes to be warm. I start my inoculation beteen 40c-45c and then let it free drop. However I the kettle wrapped in blankets on a mat under the stairs. However in Winter this is a pretty cool part of the house. I dont bother with a heat source etc, as this method has worked each time...so happy to keep doing it as it is pretty simple.
What is your grain bill? Are you standard mashing/sparging etc and collecting the wort. Do you heat to pasteurize first or do a short boil? Once you have dropped the wort to around 45c and lowered the pH with Lactic Acid (to around pH 4.5) then add a fair amount of CO2 to the kettle and cover with gladwrap and the pot lid.
You should get a drop in pH within the first 24hrs but I have always left it for 48hrs and not checked progression. But my latest dropped to pH 3.2 in 48hrs. 23Lts of 1.042 with 5 caps of this Lacto.
Oh, how are you adding the caps? Opening them and sprinkling in or whole caps. You will need to open them if you aren't and sprinkle in the powder.
And how are you measuring your pH?
IBUs aren't important - as in to say...you dont need to add them pre-boil - leave them out if you want...or put them in for the fun of it...or add to boil post souring.
Mango sour is on my list. I have just done a Raspberry Berliner and it is tasting amazing pre-bottling.