You can do whatever you like "in house" between you and your friend if that's what you both want to do. You could play second edition rules if you wanted to, you can make up whatever scenarios you want to. That's one of the nice things about 40k.
However, the new codex would (presumably) be more geared towards 7th edition, and also the old codex FAQs wouldn't necessarily make any sense with the new rules (and also aren't available any more). Also the models in the old codex would be "costed" for 7 th edition rules thereby possibly making something very powerful for the points it costs.
Also you could be doing a "dis-service" to your friends if they get used to playing your Nid army, and then go to play another Nid army from the new codex which is pointed differently and works and plays differently.
I agree that's it's a shame the mycetic spore went, but it did. Also, the Doom - it was a complete non tactical, under pointed unit that took no skill to use. Again while it's a shame it disappeared, it needed changing.
It would also mean you can't use the Harpy or the Exocrine which are both awesome units.