As for the use of liquid carbon with GrowBeams, I have a dou GrowBeam 500 (that's 2 x 500) over a 25 usg tank (30 inches x 12 x 16 inches deep) with Fluval Substratum, and low concentrations of water column ferts, no CO2; and I'm able to grow crypts (retro spirals and wendtii 'bronze') with Glossostigma. Granted I have no livestock, and I dose the liquid carbon at 4times the Seachem dose rates (I use matricide, not Exel). So neither the liquid carbon nor the lights are limiting.
As for the rails, they come in specific lengths, and need to be cut to fit your tank size.
I have copied and pasted the following from LED Aquarium Light, Lighting; Reef, Planted; AquaRay AquaBeam, GroBeam - and perhaps this issue can be addressed by the manufacturer themselves. They're in the UK aren't they? Call them. But go down the page on the link and you'll see the following. Note they give the overhang of the MountaRay as 5.75", that it best fits a rimless tank, that the 19.3" rails along with the MountaRay brackets will fit most 24" tanks (yours is 60 cm or 23.6"), and that cutting with a hack saw is a possibility.
Also, my 500s are exactly 19" long, but.... they come with mounting brackets which can be used to mount to existing hoods or to diy canopies at no extra cost.
So from the horse's mouth at Americanaquariumproducts.com:
"MountaRay (SINGLE for use with Mini 400 & 500)- $29.99
(Includes 4" mounting rail for Mini 400/500 only)
• Each MountaRay has an over-hang of 5.75" (146 mm);
• Currently the MountaRay is best fit to rimless tanks, so some modification is required for others such as cutting the trim;
• Rails are designed in length to include the MountaRay (with some cutting of rails);
The 42.9" Rails along with the MountaRay set will fit most standard 48" aquariums, the 31.1" rails will fit most standard 36" aquariums, & the 19.3" rails will fit most standard 24" aquariums
• ONE Rail is required for the 500, 600s and Mini LEDs while TWO rails are required for the 1000 and 1500 tiles
• Rails can be cut with a hacksaw as these rails are all oversized to fit as many aquariums as possible
• Rails are NOT necessary for hood/canopy mounting only as a means for an open mounting over the aquarium or with MountaRays"
Also check out the PDF included in the link which I retrieved here. They give several pictoral examples of how to mount their products.