Can't help you on what's easiest to breed, but I've kept both threadfins and gertrudes and enjoyed both for totally different reasons. Gertrudes are great little fish, but they ARE little. IMO, they'd be totally "lost" in a 40g tank with larger fish like the zebra danios. They "shine" best in nano tanks where they can be seen. A group of threadfins would fit the tank size much better, imo.
I've never had or heard of a forktail (assume you're talking furcatas) rainbow getting aggressive or territorial. I've heard that of some of the larger rainbows, but not the little guys. They're still little (I use them in nano tanks) but bigger than the gertrudes, so I think "the look" would still be ok if you had a sizeable group of them. Otherwise, I'd stick to the threadfins on a tank that size. I think any of the blue-eyed rainbows are going to have to somewhat "compete" as far as aesthetics with the danios because they are of similar shape and color but the danios are larger.