As you have found out dwarf seahorses cost a fortune in the UK and are extremely difficult to get hold of right now.
In theory the GCG could be ok in there but I personally wouldn't want to risk it. I would just keep it as an invert tank with some zoa's, etc and some smaller shrimp/crabs.
With regards to the lights you could look for a blue LED unit. How good are you are DIY? It would be very easy to make a cheap LED unit (not talking about a higher powered LED unit but one to supplement a main light source).