I was thinking of adding a bug or fighting but not sure who to pick the rest of my team is Torterra, Floatzel, Honchkrow, Alakazam and Skuntank. Also I only want to use Pokemon I can catch in-game and before the national dex.
Easy. If you have the Hm strength, then you can go into the secret entrance under the cycling bridge and get a gible. Sure it is a level 17 when you get it. But, you will have an earthquaking master on your team. Go to youtube and search "how to get gible in Pokemon diamond"
No doubt Heracross. Why pick only a bug or fighting type when you can get both in Heracross? You can find it in honey trees. (Second hardest Pokemon to find in the honey trees) If you don't want to evolve something (Machoke) via trading, then Heracross is Golden. Also the TM's it can learn (Stone Edge, Rock Slide) can destroy any flying/fire type looking for an easy time with this Pokemon.