'Codex' hormagaunts can't, but as mentioned earlier tyranids evolve very rapidly, over the coarse of as little as one or two generations, and gaunts are one of the most mutable genotypes, so you should feel free to introduce a lot more variation than occurs in the codex, versions of ranged bioweaponry included.
Wrong. This is a very common misconception with Tyranids. Most people think that they can evolve their bio-weapons, when infact the bioweapons themselves are symbiotical creatures modified and birthed on the hiveships then infused into the creatures as they are forming on the hiveship.
Another interesting point that is rather interesting is that, looking at the models and most of the older drawings, the scything talons themselves seem to be an infused symbiote, rather than their original limb end. You can see this clearest on the carnifex scything talon:

The original limb end would probably look most like a genestealer's rending claw, but I suppose that since the hormagaunts have been engineered to carry long talons, the hormagaunts that hatch out of the eggs will have talons too, but probably slighlty more curved and wider. This will probably make it slightly less efficient, but still very deadly.
On the subject of other things that could hatch out of eggs, genestealers or rippers are other possibilities.
I'm sure I've read somewhere about genestealer eggs, but I'm not entirely sure, you may have to look into that. I know that they can implant eggs into a host but I think the eggs then also absorb traits of the host. But maybe if eggs were found inside a genestealer's body, those might be able to hatch into genestealers.
Rippers I know can, in danger, burrow underground and cocoon themselves. Once cocooned they can hibernate for an almost endless amount of time. It is possible that what appeared at first glance to be eggs were actually ripper cocoons. But as well as these cocoons I'm sure I've read somewhere about ripper eggs.
Hope this helped...