I doubt you consider me more helpfull than RTR, but then I have not thought of myself ever as being more helpful than RTR so here goes anyhow.
#1 for clarification, limestone, crushed coral etc. are all forms of calcium carbonate and all will work. basically the finer it's ground and the purer the make-up the faster it will tend to dissolve. Most (maybe all) forms of calcium carbonate will slow or quit dissolving at a ph of roughly 7.6- 7.8. so there are some limiting factors with using calcium carbonate. one thing is for certain, if you have good amounts of CaCo3 in your tank it will be very stable at whatever level the Caco3 quits dissolving at. With Cichlids (assuming rift lake fish) copious amounts of any Form of CaCo3 will be a benefit.
#2 Baking soda is Sodium Bicarbonate, which is mostly carbonate and only a small amount of sodium. Noy by any stretch is baking soda mostly sodium. It is unlikely that the spinach would absorb quantities capable of helping you much, and even if it did. the process would be painfully slow, and you would be limited in your ability to do water changes. It is unlikely that your fish could consume enough baking soda blanched spinich to counteract good maintenance.
#3 baking soda is a viable way to raise PH, but it is fast and therefore should be used with caution. Unlike CaCo3 the Baking soda will raise well beyond what is desired for your fish. I do not know the limits to what it will do, but I haven't found them even when mixing concentrates to dose my tank.
#4. Salt For anyone who is confused is NaCl, and although baking soda and Salt both contain Sodium they are not really that similar as chemical compounds. That would be about the same as saying Calcium Chloride (limestone) and Sodium Cloride (salt) are simlar compounds because they both contain Clride.
Now since you are talking about cichlids, (I am still assuming African) then as with most fish the Ph is not as critical as the solids levels in the water. GH is a decent indicator of solids (far from perfect but still useable) If you don't want to use pre-made cichlid salts (not to be confused with Table salt in it's variuos forms) then concentrate on Calcium and magnesium levels in cunjunction with Kh levels. The CaCO3 will add a bit of calcium, but stops dissolving at higher Ph levels, so CaCl (Ice melter) or Ca2SO4 will increase calcium, and MgSO4 (epsom Salts) will increase magnesium. Cichlid salts as packaged are primarily these mineral salts with additional trace elements. it isn't too difficult to duplicate the rift lakes well enough with your own mix.
I am by no means a cichlid expert or a rift lake expert, but do have some experience with the chemical make-ups of hard water including the rift lakes. Others Like RTR with actual experience may be able to add to or correct this information.
Dave