I've kept GBR's, corys and tetra's in 24 degrees before quite happily - I've always found temperature for most community fish isn't an issue, as long as its stable.
GBR's are beautiful fish but as others have said though sensitive so worth giving it a few months like Mark said. I'd keep the betta in a tank on its own - easy to pick up a cheap small tank for him, you could buy second hand fish, filter and tank with change from £20. £25 inc the betta, good kitchen side tank that.
What size is your main tank? Will come up with some suggestions.