You answered my question asking for details on how your potting soil is failing with the following (bullet points mine):
- Tall spindly [stems]
- Crumpled and twisted leaves
- Slight necrosis,
- Pale leaves and stems
- Red leaves and stems.
You are currently feeding with "calcium, phosphorus and general FPJ" (another name for anaerobic compost tea, I think).
My only recommendation is to get a soil test done on your potting mix because you're simply adding a witch's brew of stuff which you have no idea will work or not. For example:
- You should stop adding phosphorus, as P deficiency manifests as stunted smaller stems, not taller, and leaves that are darker green than normal. Your tall and spindly stems and pale leaves contradict this, meaning that the phosphorus content is fine. Nitrogen deficiency also acts the same as phosphorus deficiency, so your nitrogen is at least normal, but could be high.
- If by "necrosis" you mean a bit of leaf scorch around the edges of leaves, then your plants could be suffering from potassium deficiency, although this is rare.
- Adding calcium is likely messing up the soil because too much calcium can deplete magnesium under alkaline conditions; this is an essential chemical for photosynthesis (this could be causing the pale leaves, perhaps). Too little magnesium can also lead to a phosphorus shortage even in soils that have adequate phosphorus.
- Dead spots on leaves can be caused by a number of factors, including manganese deficiency (probably not in this case) and zinc deficiency, both are more prevalent on limed soils - or soils with too much calcium! They can also be caused my more than a few fungi.
- The crumpled and twisted leaves sound more like a fungus or disease than something deficient in the soil. This makes sense since you're not sterilizing your potting soil before using.
Comfrey juice will have N,P, and K in it, but if your soil isn't deficient in these elements (and it certainly doesn't appear to be deficient in any of them), then adding it won't "charge" your soil.
EDIT: Also, the sugar in the "FPJ" could be an excellent source of food for non-beneficial bacteria and fungus, which could also be a factor in the poor performance of your plants. END EDIT
This analysis is, of course, incomplete and may be inaccurate because I don't have all the information needed, but I hope it shows you why you need to get your soil tested.
Without knowing the EXACT COMPOSITION of your soil you cannot accurately add amendments to it. Unless you get lucky, your attempts to fix your problems are unlikely to end well.
In answer your initial question: "Is it true that you can't add too much natural fertiliser... to a potting mix as opposed to synthetic fertiliser?" the answer is No, that is not true. The source of the fertilizer is irrelevant; it's the contents of the fertilizer that matters. Get a soil test.