Clown loaches will tey and eat the eggs, but likely with no success. GT's are fantastic parents so i wouldn't worry too much.
Larger tetras, danios and thinks like tiger barbs make great dithers. So you could use anything like that really.
I wouldn't do two male GTs as they fight constantly, resulting in the weaker ones death. I have a pair of GTs with two pairs of cons, a FM, an RD and two O's in my 180 atm. For the time being they all get along fine. So the FM shouldn't cause too much of a problem. Just keep an eye on them and be prepared to move the FM should thinks get aggressive.
considering the tank size i have come down to only a pair or GT... 2 pairs of clown loach and few dithers... wouldnt mind a FM to be there provided i get a goood specimen...
how big is ur RD? ive been thinking of getting one... but not for this tank... i have an 195 in that i have 6 O's, 4 Pacus, 1 Giant Gauramy, 1 Green Texas(one aggressive MOFO