Red head tapojos do fine in harder water and are quite a resilient species. As long as the tank water is clean and stable a group of these would be very suitable for your proposed tank. I have 8 red heads in my tank at the moment and they are lovely fish.
If they breed, they are mouth brooders and that nevers gets old to witness. Lots of bogwood, some leaf litter, twigs and other botanicals would be complimentary decor, scattered over a fine sandy substrate.
Regarding the spilurus, yes if you go with the 60" tank I think you could add a pair.
They are a nice, quite well behaved cichlid by c/a standards. As I mentioned earlier in this thread, there are never guarantees. It's always wise to have a back up plan if things get aggressive. A tank divider on standby can be a valuable, temporary tool for saving a fish on the receiving end of unwanted aggression.
Expect some chasing and displays of bravado, that's what cichlids do sometimes. Meeki are notorious for their gill flaring displays. This is all normal and expected. It's time for intervention when a fish is constantly hounded.
I think with the fish your interested in you shouldn't get much trouble.