I'm sorry to hear that the ram died.
All the colour varieties of ram behave the same; having two gold and a blue, for example, just means you have 3 rams. Bolivian rams, being a different species, will not mix with rams.
You have to be careful with cichlids. Males defend a territory on the bottom of the tank. They will not allow males of the same species into it, and may also chase away males of similar species. So having M. ramirezi (ram) males and M. altispinosus (Bolivian ram) males in the same tank may not work long term. And just buying more rams won't guarantee anything either. If you got more males, the current males would object. if you got more females, they may not get on, and could kill each other. Rams, and Bolivian rams
must choose their own mates. The usual way is to buy 6 rams or 6 Bolivian rams as juveniles, let two pair up then rehome the other 4. But this won't work here because you already have rams in the tank.
What you really need to do is work out exactly what you have. How many male and female rams, and how many Bolivian rams, though it will be impossible to say whether those are male or female. Then decide which you want. You will have a few options - keep the bolivian ram(s); keep the 'ordinary' rams; or even part exchange all the rams you currently have when buying more fish for your new tank, then get half a dozen juveniles of either rams or Bolivian rams, let them pair up then take the rest back. With non-Bolivian rams, you can mix colours.
As for your sand, I'm sorry to tell you but white is just about the worst colour from the fish's point of view. If it is bright white it will stress the fish because they have evolved over, and therefore expect, dark substrates. This is why fish look dark when we look down on them so that they blend in with the substrate (river, lake bottom) and are less easily seen by predators. Fish are also pale underneath so they blend in with the sky, so that predators lower down in the water can't see them properly.
I would be inclined to mix your current white sand with a darker sand in the new tank so it comes out a dark and white speckled mix.
Provided the slate has no sharp edges whatsoever, it could look good say piled up in one corner. Our member
@Matt has an amazing aquascaping website to give you some ideas

When we give numbers for pH, ammonia, nitrite and nitrate, those are all ppm as measured by our test kits. Just about all test kits contain those 4, but some contain others as well. The 4 are the ones you need while the extras are really optional. It depends how much money you are willing to spend. Some of the extensive kits are mainly aimed at people who have a lot of live plants and need to know the levels of all sorts of things for their plants.
I have the API kit. That contains 2 pH testers but as you are in Scotland with soft water the chances are you also have pH below 7.6 so you wouldn't need the high range pH bottle.
All liquid nitrate testers have more than one bottle - usually 2 or sometimes 3 bottles. The instructions say to shake one of those bottles very well, and also the test tube just after the drops are added. This is because one of the reagents isn't actually soluble and it settles on the bottom of the bottle. The shaking is necessary to get it spread evenly through the liquid, and failure to shake is the most common cause of false nitrate readings.
Your hardness of 36.43 - is that mg/l CaCO3 or mg/l Ca?
Water companies could use any of half a dozen units. Fishkeeping uses only 2 units - ppm, which is the same as mg/l CaCO3, and dH, which is another name for German degrees.
So if your 36.43 is in mg/l CaCO3, that is 36.43 ppm and 2 dH. But if it is 36.43 mg/l Ca, that converts to 92 ppm and 5 dH. While both of those are classed as soft, you do need to know if your water is very soft or just soft.