Yes, agree with the advice given. Adding the bicarb (Baking Soda) during the fishless cycle is fine and is the way to get the pH up in the 8.0 to 8.4 range that you want for maximum growth of the autotrophic bacteria. If you're only making it up to 7.5 then dose more bicarb. It will not harm your tank as it should all go out at the big water change before fish. It is not the method of choice if one wishes to raise parameters after fish are in the tank though, so don't continue it past cycling.
Fluctuations in pH, as described, are often not a problem as it is the underlying change in mineral content (as measured variously by water hardness tests) that can bother the fish more profoundly. It's important to never attempt to judge what your water chemistry will be like in the finished tank based on what you are seeing during fishless cycling. The tank may not be the same at all. Instead you just have to get a good cycle (with qualifying week and all) and then do the big water changes, get fish and begin to watch how the normally operating tank settles in. It's good practice to continue to record good notes in your aquarium notebook throughout the first months to help you get a feel for the water chemistry of the running tank as based on real data, not just the whims of your memory.
The the special lake cichlids you are hoping for, the first hope will be that the tap water settles in at a high enough mineral content and you are able to take care of them that way. If it fails to be high enough then it may be that a small mesh bag of crushed coral in the filter will be the way to go, but of course this will be more maintenance work for you ongoing.
~~waterdrop~~