Hi, Lario, welcome to TFK!!!
I don't have experience with the German Blue Rams, but I have kept a shoal of Bolivian Rams for well over a year now, and my understanding is that their care is very similar.
Your Ph is probably okay, but I can't recommend that you keep them over gravel (though some people do). Sand is the way to go with these little gems, and I don't think setting aside a corner of your tank for sand is going to be enough. These fish feed by sifting through mouthfuls of sand, and when they spawn they use their mouths to dig pits all over the tank for their brood (and I mean that quite literally. A single spawning pair of Bolivians nearly uprooted every plant in my 55g!). Even their name Mikrogeophagus means 'Little Earth Eater.' I don't think that they can be truly comfortable over gravel. . .
In a 24" tank, I would only be comfortable keeping a single ram, or a bonded pair. From what I've read, they do live singly in nature, except when it comes time for breeding, so a lone fish ought to be just fine on his own - and many people do keep rams singly. You may not see the gorgeous coloration or the engaging behaviors that you would were they kept in a group, or as a pair. When I've kept Bolivian Rams solo, they aren't nearly as bright as when they're kept with others of their kind because they aren't vying for dominance and 'showing off' for one another. But finding a pair of rams is not as simple as putting a male and a female in the same tank, either. These fish often form bonds for life with their mates, and prefer to choose their partner - just as you would! I have read many cases where a male and female put into the same tank do not get along, and although in some cases they do mate, it seems to end badly more often than not. Most people recommend getting six rams as juveniles, and allowing them to form their bonds naturally as they grow, re-homing those that have not bonded. But you don't have room in a tank of that size to do this. . . so you'd probably be better off sticking with one pretty male.
Both GBR and BR are sensitive - both fish require very stable water conditions in a well-established tank, and do not react well to shifts in parameters, so you'll want to be certain that your tank is very stable before bringing one home. From what I've read, the excessive breeding of GBR for particular traits and color-forms has made these fish very weak and disease-prone - one of the reasons I chose to go with the tougher Bolivians instead (I'm still new to fish-keeping!). If you do decide to get GBR, I'd advise that you stick with the more natural forms, and avoid the "man-made" varieties, like electric blue, yellow/gold, etc. as I've read that selective breeding has made these types even more fragile, and of course never buy 'balloon' anything. I'm not sure where to tell you to order them online, but if you can find a local breeder, that may be your best bet, and maybe you could find someone who would be willing to sell you an already-bonded pair!?
I hope this helps you out somewhat! Again, I don't keep GBR, this info is based on what I know about Bolivian Rams, and what I've read about both. So please take my words with a grain of salt, and continue to do your own research before you decide to bring your babies home!
Best of luck to you!
ETA: I have several videos up of my Bolivian shoal, if you'd like to see where my experience stems from, here is the link
to my YouTube channel. . .