Well, it depends. After you beat Diamond, you said you'll transfer your Pokemon from FireRed to Diamond. So, I think that you should catch all the Pokemon you can in FR, and transer them to Diamond, like I'm sure your doing. And after you transfer ALL the Pokemon to Diamond, you could restart the game to get all the starters.
That's what I'm planning on doing on my Emerald. I caught everything I could, and I'll transfer them over to Diamond. Then, after the transfers are complete, I'll restart it to get Torchic and Mudkip, the other Lati, and the Skitty I couldn't evolve because I only had one Moon Stone and I spent it on Jigglypuff. Now, you could restart your FR after everyone's transferd to get the other starters and legendary dogs.